Snapshots and forks of storage systems using distributed consistent databases implemented within an object store

ABSTRACT

A method is described that includes providing a snapshot counter for a storage system implemented with multiple distributed consistent database instances. The method further includes recognizing the taking of a snapshot of the storage system by incrementing the snapshot counter from a first snapshot counter value to a second snapshot counter value. The method further includes, in response to a first change for one of the distributed consistent databases subsequent to the taking of the snapshot, performing the following: saving state information of the distributed consistent database as the state information existed prior to the change and associating the first snapshot counter value to the state information. Associating the second snapshot counter&#39;s value with the distributed consistent database having the first change. Another method for the taking of a snapshot of a distributed consistent database is also described.

RELATED CASES

This application is a continuation of and claims the benefit of U.S.patent application Ser. No. 14/485,585, titled “ SNAPSHOTS AND FORKS OFSTORAGE SYSTEMS USING DISTRIBUTED CONSISTENT DATABASES IMPLEMENTEDWITHIN AN OBJECT STORE”, filed Sep. 12, 2014, which is incorporated byreference in its entirety.

FIELD OF THE INVENTION

The field of invention relates generally to the computing sciences andmore specifically to snapshots and forks of storage systems usingdistributed consistent databases implemented within an object store.

BACKGROUND

FIG. 1 depicts three primary types of storage systems. As will be mademore clear immediately below, what differentiates each of the storagesystems from one another is the mechanism that is used to access thestored information.

FIG. 1a shows basic object storage. In the case of an object store, arequestor 101 requests access to a specific item of stored information(“an object”) from the object store 102 by way of an “object ID” 103. Anobject ID is a uniquely identifying reference of a specific object withthe store (e.g. a randomly assigned alphanumeric character). Anothertype of store that conceptually has a large degree of overlap with anobject store is a “key-value” store. By definition, a key-value storemaps individually stored information (i.e., the “values”) to individualkeys that are uniquely assigned to the values. A specific set of storedvalues are accessed by providing the store with the key for thosevalues. Thus, the “key” corresponds to an “object ID” and the “values”correspond to an “object”.

FIG. 1b shows file directory based storage. In the case of filedirectory based storage individual items of stored information are keptin files 104_1 through 104_N. Files are organized into a directory whereany sub-directory 106_1 through 106_M can include one or more files orone or more lower sub-directories. A specific item of information isobtained by accessing its file. The file is accessed by articulating thepath through the directory that leads to the file. For example, in theexemplary directory of FIG. 1b , file 104_3 is accessed by articulating“C/FOO/BAR/NOTES”.

FIG. 1c shows block based storage. In the case of block based storage,the storage resource is viewed as being broken down into contiguousblocks 107_1 through 107_N. A specific item of information is accessedby identifying the block where it resides. A common type of blockstorage is a “thin provisioning” system in which a block is identifiedby an offset within a larger “logical” storage resource. Thinprovisioned systems have been traditionally used to more efficient useof physical storage resources as unused block segments do not occupyphysical storage space. For example, a logical file (e.g., ascontemplated by an operating system) may include active data portions or“stripes” where actual data resides and “unused” portions where nosubstantive data resides.

The unused portions may correspond, for instance, to regions that havenot been written to yet. The “real” data that has been written to iswritten in “stripes”. For example, if FIG. 1c represents a logical filethat spans offset 80 through 106, stripes 80-87 and stripes 100-106contain real data and are stored in physical storage while offset values88-99 are understood to contain 0s that are not actually stored inphysical storage.

While FIGS. 1a-1c depict core physical storage solutions by themselves,FIG. 1d shows a fundamental usage case or implementation of any of thesecore storage solutions. FIG. 1d shows a traditional “database” storageimplementation. As observed in FIG. 1d , the traditional databasestorage implementation includes a database interface 108, an indexinglayer 109 and a storage layer 110.

The database interface layer 108 accepts queries to the database storage(e.g., according to some type of predefined structured query language(e.g., SQL) or other query format). A common feature of queries is thatthey often identify data by its content rather than a particular addresswhere the sought for data is found. For example, a query submittedthrough interface 108 might ask for all email addresses having aspecific string (e.g., “@abc.com”). It is the role of the indexing andstorage layers 109, 110 to actually find the targeted information.

The indexing layer 109 exists to speedup lookups into the storage layer110. As a point of comparison, without the indexing layer 109, a queryfor a particular item of information within the storage layer 110 wouldbe accomplished primarily by scrolling through each item of informationkept by the storage layer 110 until the sought for information wasfound.

The function of the indexing layer 109 is similar to the index at theend of a textbook. With input criteria specifying the sought afterinformation (e.g., “@abc.com”), the index returns with one or morepointers or other information useable to fetch the specificallyrequested information from the storage layer 110. Thus the indexinglayer 109 can be viewed as a mechanism for effectively searching thecontents of the underlying storage layer 110.

The storage layer 110 corresponds to the resources used to actuallystore the information and can be implemented with any of the storagesystems discussed above with reference to FIGS. 1a through 1 c.

FIG. 1e depicts the implementation of a relational database. As isunderstood in the art a relational database is typically contemplated asa collection of tables. The individual rows of the various tables areviewed as the records that are kept by the relational database. Thecolumns of a particular table are used to keep the various data itemsfor a particular row. For example, referring to table 120, if each rowrepresents a particular person, a first column may be used to keep theperson's name, a second column may be used to keep the person's phonenumber, a third column may be used to keep the person's email address,etc. Rows whose data content are defined by the same set of columns arelogically viewed as being kept within the same table.

Typically, one column of a table is identified as the “primary key” usedto identify a particular row within the table. For example, continuingwith the example above where each row represents a person, one column ofa table may be used for a “userid” that uniquely identifies the person.With the userid for a particular person, the row in the table for theparticular person can be easily fetched. In this sense, the userid alsoacts as a primary key for accessing the table as well. Here, a primarykey may include a combination of an identifier of the table and anidentifier of a specific row within the table. The columns of a tablemay also include the primary keys of (rows of) other tables to establish“relationships” between records. For example, if the columns of table120 table keep the personal information for specific people, one ofthese columns may include the primary key for another table 121 thatkeeps records on the employers of these people.

Thus, if a logic operation desires to know specific information about anemployer of a particular individual, the logic operation may firstaccess the person's record from table 120 with the userid for thatperson and obtain the primary key for the row in table 121 that containsinformation about the person's employer.

Relational databases can also be “queried” for specific information. Forexample, the relational database discussed above could be queried toprovide the names of all people who have a specific area code in theirphone number. Here, the first table 120 would be accessed, all therecords having the sought for area code would be extracted and theinformation from the name column of these extracted records would beprovided as the query result.

Note that an indexing layer 122 could be used to speed up the queryingprocess. That is, rather than simply access the table 121 and scrollrow-by-row through it for records having the desired area code, instead,an index layer 122 could be built on top of the tables 120, 121 that isdesigned to provide the primary keys of people based on their area code.Here, the index 122 can take the form of a B+ tree whose nodes areassociated with specific area code numeric ranges with a narrower rangebeing specified moving down through the tree. Eventually the leaf pagesof the B+ tree are reached that provide specific primary keys forspecific people having a specific area code. In a basic approach thereis a separate index for every column in a table so that any query forany item of information within the table can be sped up as describedabove. Of course, any changes to the records in the table will need tobe reflected in the index.

FIGURES

FIGS. 1a through 1e pertain to prior art storage technologies;

FIG. 2 shows an improved storage technology that provides for quotas,thin provisioning and relational databases with distributed consistentdatabase technology;

FIG. 3 shows an exemplary KVS layer;

FIG. 4 shows a distributed consistent database;

FIGS. 5a-5d show basic access behavior for a distributed consistentdatabase;

FIG. 6 shows a methodology that outlines the basic access behavior of adistributed consistent database;

FIGS. 7a-7e show distributed consistent database behavior when changesare made to the distributed consistent database;

FIG. 8 shows a methodology outlining the behavior of a distributedconsistent database when changes are made to the distributed consistentdatabase;

FIG. 9 depicts an exemplary use of a distributed consistent database toimplement a file directory;

FIG. 10 depicts an exemplary use of a distributed consistent database toimplement a thin provisioning block device;

FIG. 11 depicts an exemplary use of a distributed consistent database toimplement a quota system;

FIG. 12 depicts an exemplary use of a distributed consistent database toimplement a relational database;

FIG. 13 is a schematic depiction of various manners in which thearchitecture of FIG. 2 can actually be implemented in practice;

FIG. 14 shows a computer system;

FIG. 15 shows an improved storage system that includes snapshotcapability;

FIG. 16 shows an exemplary depiction of snapshots being taken of astorage system;

FIG. 17 shows an exemplary depiction of two object proxies;

FIGS. 18 a,b,c each show methodologies that pertain to snapshots of astorage system.

FIG. 19A through 19I demonstrate mapping table and object manipulationsas a function of global forking, global snapshots, local forking andlocal snapshots;

DETAILED DESCRIPTION

1.0 Overview

FIG. 2 shows an embodiment of a new and versatile storage architecture200. As observed in FIG. 2, the new architecture includes an object orkey value store (KVS) 201, a distributed database management system(DDS) 202 (implemented with separate DDS instances 202_1 through 202_N)and a connectors node system (CNS) 203 (implemented with separate CNSinstances 203_1 through 203_N).

At a high level perspective, the KVS 201 can be viewed as the principlestorage resource of the system. In various implementations the KVS isimplemented as an “object store” that stores “objects”. Objects andobject stores are well understood in the art. Objects are units offundamental storage in an object store. Each object is assigned its ownunique (e.g., random) identifier that uniquely identifies itscorresponding object. This particular type of access is distinguishingfrom other types of storage systems such as file systems (whosefundamental unit of storage, a “file”, is identified with a directorypath) and block storage systems (whose fundamental unit of storage, “ablock” is identified with a numerically restrictive offset).

Here, as will be discussed at length below, in an embodiment, the KVS201 is implemented as an object store having a Chord-like distributedhash table access mechanism. The combination of a Chord-like distributedhash table access mechanism with an object store provides for certainadvantages for large scale storage systems (principally: intelligentrouting resulting in reduced message passing, parallel accessingcapability and the ability to keep meta-data along with the storedinformation).

As described in the background, however, key value store systems employthe use of a key or object ID as the primary mechanism for accessing thedata. This means other storage types, such as directory storage or blocktypes of storage, are not natively implemented on the primary KVSstorage 201.

The DDS 202 therefore is added as a database management layer above theKVS 201 to provide interfaces 206, 207 of the KVS 201 that permit theKVS 201 to be used as file directory or block based storage system. Thefile directory interface 206 essentially acts as a translation layerthat converts an access specified in the form of a directory into anobject ID for the KVS 201. Likewise the block storage interface 207 actsas a translation layer that converts an access specified in the form ofan offset (or other type of block specific access) into an object ID forthe KVS 201. Practically then, the KVS 201 can be used directly as a keyvalue store or, indirectly as a directory based storage (with adirectory interface 206) or indirectly as a block store (with a blockinterface 207). If the KVS 201 is sufficiently large, one or more ofeach of these different types of storage systems may be simultaneouslyimplemented.

In an embodiment, both of these interface layers 206, 207 are built witha special mechanism, referred to as an “distributed consistentdatabase”, for effectively imposing structure or organization into theKVS 201 Each instance of a distributed consistent database includes ahead object for the structure to be imposed. The head object has amapping table that defines the hierarchy of the structure and itsimplementation in KVS. In an embodiment, the mapping table: i)identifies a root page that represents the apex of the hierarchy; ii)identifies the intermediate and leaf pages that reside beneath the rootpage; iii) maps the “page ID” of the intermediate and leaf pages tospecific object IDs within the KVS 201. Leaf pages contain object ID(s)(or information from which object ID(s) can be determined) for theinformation within KVS that the structure was accessed to obtain.

Individual users can be provided, through interface 213, with their owndistributed consistent database instances for their own individualstorage. For example, a first user may be provided a first distributedconsistent database instance for storage of the first user's content anda second user may be provided with a second distributed consistentdatabase instance for storage of the second user's content, etc.However, multiple distributed consistent database instances can also becoordinated together as fundamental kernels in the construction of asingular, extremely large capacity storage solution. In one approach, aunique distributed consistent database is instantiated at each node of astorage system's hierarchy (e.g., each directory, each sub-directory andeach file of a file system) to effect extremely large and easily managedstorage resources at each node of the system.

As such, as observed on FIG. 2, the file directory interface 206 usesthe distributed consistent database technology 213 to effect extremelylarge scale file directories, and, the block storage interface 207 usesthe distributed consistent database technology 213 to effect extremelylarge block storage systems.

Moreover, the behavior of the distributed consistent database naturallylends itself to the implementation of “functional bricks” that can runon top of any storage system implemented within the KVS 201. An exampleof one such functional brick is a quota policy 209. Another functionalbrick is a relational database 210. The functional bricks 209, 210 andinterfaces 206, 207, 213 can be used in various combinations to effectvarious storage systems.

With the DDS 202 and the CNS 203 a wide range of different storagesystem interfaces to end-users 205_1 though 205_M. Here, an “end-user”or “user” or “requestor” is any entity that makes use of the storageresources in KVS 201. Examples include an application software instance,an application software process, a client computer instantiated with anyone or more of these software instances/processes, an organization suchas a corporation, etc.

With direct access to the KVS 201, the CNS 203 is able to providevarious object store connectors/interfaces to end-users (e.g., CloudData Management Interfaces (CDMI), Simple Storage System (S3), etc.).With access to the file directory interface 206 provided by the DDS 202,the CNS 203 is able to provide any directory file systemconnector/interface to end-users (e.g., Network File System (NFS),Common Internet File System (CIFS), File System in User Space (FUSE),etc.). Likewise with access to the block storage interface 207 providedby the DDS 202, the CNS 203 is able to provide any block storage systemconnector/interface to end users (e.g., iSCSI, FC). Again, any/all ofthese different storage solutions may simultaneously be implemented onthe KVS 201 as the actual physical storage resource.

2.0 The KVS System

FIG. 3 shows a depiction of an embodiment 301 of the KVS system 201 ofFIG. 2. As mentioned above the KVS system 301 can be implemented as anobject store having a Chord or Chord-like distributed hash table accessmechanism. Such a KVS system 301 can readily be implemented with lessexpensive disks. Moreover, due to the fact that any of its storage nodescan receive input queries, it essentially has a parallel accessingstructure which can be used to diminish any points of contention whenwriting in parallel and offers good overall parallel I/O performance.

As will be described in more detail below, in an embodiment, the KVSsystem 201 has the following properties: 1) it is composed of storagenodes; 2) it understands specific verbs to create, read, update anddelete data; 3) it manages versions and other system attributes perobject; 4) it understands specific verbs to manage reservations whichprevent concurrent writers to overwrite an object; 5) it forbidsconcurrent writing to and reading from the same object; 6) it permitsconcurrent multiple reads of the same object; 7) it does not require astrong consensus algorithm to operate; 8) the system may havehierarchical levels in which an attempt is made to place objects thatare more likely to be accessed in a higher level than objects that areless likely to be accessed (e.g., caching); 9) multiple replicas of asame data object may be stored on different storage nodes to ensurereliability should any particular node go down.

The same keyspace is used for both keys and actual storage nodes wherethe entirety of the keyspace is envisioned as a logical “ring”. Somelocations on the ring 303_1 through 303_R correspond to actual storagenodes. Each storage node may receive queries from one or more DDSinstances. Remaining ring locations correspond to the respectivekeyspace IDs or “object IDs” for the data objects that are stored on thestorage nodes. Thus, the object ID for a stored data object essentiallydefines its storage location on the ring.

According to the routing scheme of the system, object IDs are mapped totheir successor node. For example, the object IDs within range 306 aremapped to node 303_1, the object IDs within range 307 are mapped to node303_2, etc.

Each storage node has its own associated routing function, referred toas a finger table. FIG. 3 depicts node 303_2's finger table 311 as anexample. Finger table 311 identifies the range 307 of the object IDs 309whose corresponding objects are stored locally on node 303_2. Node303_2's finger table 211 also includes routing information 310 thatdirects a query for any object ID outside its local range 309 to anothermore appropriate node downstream along the ring. As such, when an objectID is presented to the finger table 311 of node 303_2, the locallystored range 307 information is referred to and the requested dataobject is immediately returned if it is stored locally on node 303_2. Ifthe object ID is outside node 303_2's object ID range 307 the query isdirected further along the ring to a particular node specified in node303_2's routing information 310.

Generally, the farther a querying object ID is outside a node's localstorage range, the farther downstream along the ring the node's routinginformation will direct the query. The mathematical protocol used inconstruction of the nodes' respective finger tables ensures that thequery will “hit” the node whose local storage range covers the queryingobject ID within a limited number of routing hops around the ring.

Additionally, through a technique referred to as “replication”, a singleobject ID can be mapped to more than one node along the ring. By sodoing, multiple instances of a particular data object are kept atvarious nodes along the ring thereby ensuring reliability should a nodehaving the data object suffer a failure. When a node does fail, all thatneeds to be done to reconfigure nodal routing information is to updatethe failing node's successor to include the failing node's object IDrange and update the finger table routing information of the affectednodes along the ring.

According to one approach, referred to as “consistent hashing” each ofthe nodes along the ring will locally store approximately the samenumber of object IDs As such complex routing table reconfiguration isnot required in the face of a node failure or join.

The object ID for a stored data item may, for instance, be a randomlyassigned value or the output of a hashing function. In oneimplementation, an object ID is assigned for a data item when the datais first presented to the KVS system for storage (e.g., with a CREATEoperation). The KVS system may provide the caller of the CREATEoperation with the object ID for the data item for subsequent referenceto the same data object.

In an implementation, the “object ID” corresponds to entropy informationwithin a larger key structure that is actually used to fetch data alongthe ring. The larger key structure may include, for example, informationappended to the entropy object ID that identifies the number of replicasof the data object within the system as well as which replica thespecific key structure corresponds to.

In one embodiment, the individual data items that are stored in the KVSsystem are embodied as a “chunk” of information having not only theactual data object being stored but also meta-data containing systemand/or user defined meta data that is stored along with the actual dataobject (for simplicity the remainder of the document will refer mainlyto a data object). The system meta-data may include, e.g., time ofcreation of the data object, size of the data object, last access timeof the data object, last modification time of the data object and theversion number of the data object among other possible characteristics.The user defined meta-data can be any attribute of the data objectdefined by the user. System meta-data is updated as appropriate when anobject is accessed. User meta-data (and even system meta-data) for aparticular data object may be returned along with the data objectwhenever the data is accessed (e.g., for GET and PUT operations).

In an embodiment, the KVS system supports CREATE, PUT, DELETE and GEToperations and uses a reservation system to ensure data consistency.Here, RESERVE commands are sent to nodes to effectively place a lock onthe data item.

In the case of a CREATE operation, which is used to create a new object,a RESERVE command is initially sent to every node that will be used tostore the data object or a replica of the data object. A subsequentCREATE command for the same object is not issued to any such node untilan appropriate response is received from each node to which a RESERVEcommand was sent. In the case of the creation of a new data object, anappropriate response to a RESERVE command includes an indication thatthe object ID for the new data object is available and there are nocompeting requests for the same object ID.

In the case of a PUT operation, which is used to update an existing dataobject, like the CREATE operation, a RESERVE command with a “PUT” intentis initially sent to every node that stores the data object or a replicaof the data object. A subsequent PUT command is not issued to any suchnode until an appropriate response is received from each node to which aRESERVE command was sent. In the case of a PUT operation, an appropriateresponse to a RESERVE command includes an indication that the specificdata object exists and there are no competing PUT, GET or DELETErequests for the same data object. As part of the process of modifyingthe data object with a PUT command, the version number of the dataitem's meta-data is incremented commensurate with the writing of the newdata.

In the case of DELETE operations, like PUT operations, a RESERVE commandis initially sent to every node having the data object or a replica ofthe data object. Unlike a PUT command, however, a DELETE command isissued to a node that appropriately responds to a RESERVE command assoon as the response to the RESERVE command is received. Here, anappropriate response to a RESERVE command for a DELETE operationincludes an indication that the object exists and there are no competingPUT or GET requests for the same object.

In the case of GET operations, a RESERVE command with “GET” intent issent to every node having the data object. If the RESERVE command isresponded to favorably (which indicates that the data object exists andthere are no competing PUT or DELETE requests for the same object), thedata item is returned to the caller. Here, the fastest replica with thehighest version number is chosen for the GET operation (otherwise theoperation fails and is retried later).

In a further implementation, the KVS system may be enhanced to includeone or more supervisory nodes (not depicted in FIG. 2) that arecommunicatively coupled to the storage nodes of the ring. Thesupervisory node(s) execute processes that: 1) formally join new nodesto the ring; 2) formally delete existing nodes from the ring; 3) monitorthe ranges of object IDs stored on each node; 4) detects incorrectstorage of object IDs on a particular node (e.g., a node is storingsuccessor object IDs); and, 5) resolves any such incorrect object IDstorage.

In another further implementation, the KVS system has multiple storagetiers (also not shown in FIG. 2). For example, a first ring is used as acaching layer (tier 1) and a second ring is used as a deeper storagelater (tier 2). Here, accesses to the tier 1 layer are generallycompleted sooner than accesses to the tier 2 layer. A probabilisticoffload engine determines which data objects warrant storage in the tier1 layer (e.g., objects deemed more likely to be accessed in theimmediate time frame are identified for storage in the tier 1 layer).Various semantics between the caching and deeper layers may be exercised(e.g., write through, copy-on-write, etc.) to ensure data reliabilityand consistency.

More details pertaining to an implementation of a KVS system may befound in U.S. application Ser. No. 12/640,373 filed on Dec. 17, 2009entitled “Multipurpose Storage System Based Upon A Distributed HashingMechanism With Transactional Support and Failover Capability” and issuedas U.S. Pat. No. 842,944 and U.S. application Ser. No. 12/964,656 filedon Dec. 9, 2010 and entitled “Probabilistic Offload Engine ForDistributed Hierarchical Object Storage Devices” both of which arehereby incorporated by reference in their entirety into the instantapplication.

3.0 The DDS Database Management System (the Distributed ConsistentDatabase and Uses of the Distributed Consistent Database)

Referring to FIG. 2, the distributed database management system (DDS)202 is used to help realize different kinds of structures that can beimplemented within the KVS 201. Specifically, as discussed with respectto FIG. 2, the KVS system dictates the use of a key (object ID) to acaller of the KVS system 201. As such, in order to implement directorybased storage systems and/or block based storage systems in KVS, the DDS202, in an embodiment, offers both a directory interface 206 and a blockstorage interface 207.

As discussed above, the directory and block storage interfaces 206, 207rely on an underlying structural kernel imposed into KVS, referred to asa distributed consistent database 213, that permits for extremely largestorage systems. Distributed consistent database instances may also beexposed for direct use. Additionally, distributed consistent databasescan be used to implement other functions or applications on top of anyof the storage systems implemented within KVS 201. Examples includequotas 208 and relational database functions 210.

3.1 The Distributed Consistent Database—Structure and Overview

FIG. 4 shows an instance of a distributed consistent database. Referringto FIG. 4, a distributed consistent database includes a hierarchy ofpages 400 that are managed within a DDS instance 402. The hierarchy ofpages 400 are used to navigate accesses for a collection of objects 403within KVS 401 that the distributed consistent database is the keeperof. Thus, objects 403 typically correspond to objects containinginformation of the “user” (“customer information”) while the hierarchyof pages 400 is used to correctly identify which of objects 403 havebeen targeted by any particular request made to the distributedconsistent database.

In an implementation, as will be described more completely below, eachpage of the hierarchy 400 is kept within its own object in KVS 402.Objects containing hierarchy pages are called into DDS 402 from KVS 401,e.g., “one at a time”, as their corresponding pages are actually needed(objects whose pages “might be” needed may also be called up inadvance).

In an embodiment, a page is understood to be a collection of informationthat can be switched into memory and switched out of memory as a unit bya software program that operates on the page's information. As is knownin the art, one or more pages of information are called into memory by asoftware program, where, there is typically some limit on the size ofthe pages and/or the number of pages that can be called into memory bythe software program. In operation, to effect efficient use of memory, asoftware program will call into memory the pages it needs or believes itmight need and switches out of memory the pages it does not need orbelieves it does not need.

In practice, each “page” may be implemented as a document (e.g., an XMLdocument, JSON document or binary representation) or other constructthat can be contained within an object store object and keep thehierarchical and/or navigational logic of the access hierarchy scheme.

Thus, in an implementation, the corpus of hierarchical pages 400observed in FIG. 4 for an entire distributed consistent database aretypically not all resident within DDS 402 at any instant of time.Rather, only a subset of these pages 400 are eventually called up fromKVS 401 to satisfy any particular request. The hierarchy of pages 400can easily “expand”, however, akin to the behavior of a B+ tree, toaccommodate extremely large numbers of objects 403 that the distributedconsistent database is used to store.

As observed in FIG. 4, the hierarchy of pages 400 include a root page411, various intermediate pages 412_1 through 412_X and various leafpages 413_1 through 413_Y. Although only one level of intermediate pages412 is depicted, a distributed consistent database may have multiplelevels of intermediate pages. Each of the root page 411 and theintermediate pages 412_1 through 412_Y include the page IDs of theirimmediately lower pages. For example, root page 411 includes the pageIDs for each of intermediate pages 412_1 through 412_Y, intermediatepage 412_1 includes the page IDs for leaf pages 413_1 through 413_Z,etc. Each page may also include some form of logic and/or information tobe processed by such logic (e.g., within DDS software) that determineswhich is the appropriate next lower page for any particular request. Forexample, if a particular request targets an object within objects 403whose object ID is kept on leaf page 413_1, the logic of root page 411,in view of the request, will produce the PAGE ID for intermediate page412_1, and, likewise, the logic of intermediate page 412_1 will producethe PAGE ID of leaf page 413_1.

In this manner, any particular request is resolved to the correct leafpage. A leaf page contains one or more object IDs or other referencesfor the objects 403 within KVS 401 that are kept by the distributedconsistent database. For example, in the case of a simple read request,when the read request is applied to the correct leaf page, the leaf pageprovides the object ID or other identifier for the specific object inKVS that has been requested.

3.2 The Distributed Consistent Database—Basic Access

FIGS. 5a-5d show basic access behavior for a distributed consistentdatabase and FIG. 6 shows a methodology that outlines the basic accessbehavior.

Referring to FIG. 5a and FIG. 6, a request 510 is received thatidentifies a particular distributed consistent database and an action tobe performed on that distributed consistent database 601. The request510 may be generated externally from DDS 502 (e.g., in the case wherethe distributed consistent database is exposed for direct use) orinternally within DDS 502 (e.g., in the case where a larger databasesolution offered by DDS is invoking the distributed consistentdatabase). Actions to be performed on a distributed consistent databasetypically include any of: 1) the fetching of one or more objects 503from within KVS 501 that the distributed consistent database isconfigured to keep; 2) the addition into KVS 501 of one or more objectsinto the group of objects 503 that the distributed consistent databaseis configured to keep; or, 3) the deletion from KVS 501 of one or moreobjects from the group of objects 503 within KVS 501 that thedistributed consistent database is configured to keep.

In response to the request 510, referring to FIGS. 5b and 6, the DDS 502reads 602 from KVS 501 a “head” object 511 for the specific distributedconsistent database that is targeted by the request 510. In animplementation, the head object 511 for the distributed consistentdatabase targeted by the request is identified in the request itself 510or is readily identifiable from the request itself (e.g., the object IDis calculated from the request through some mathematical operation).Alternatively, the DDS 502 may keep a repository that tracks the headobject IDs for all the distributed consistent databases it hasinstantiated in KVS 501.

The head object 511 contains a mapping table 512 and the object ID 513for the object 514 within KVS 501 that contains the root page 515 forthe distributed consistent database. As will be made more apparent inthe following discussion, the mapping table 512 is a data structure thatcorrelates the PAGE ID of the intermediate and leaf node pages of thedistributed consistent database's hierarchy to its corresponding KVSobject ID. The root page 515, corresponds to root page 411 of FIG. 4 andrepresents the root (highest level) node in the hierarchy of thedistributed consistent database. The object ID 513 for the root page 515found in the head object 511 is used to initially fetch 602 the rootpage 515 from KVS 501.

The information on the root page 515 identifies, via “page IDs”, theimmediately lower nodes of the distributed consistent database'shierarchy that stem from directly beneath the root page. The root page515 also contains logic (or information to be used by such logic withinthe DDS software) for determining which of these page IDs is the correctpage ID for the next lower node of the distributed consistent database'shierarchy in view of the request being processed.

Referring to FIG. 5c and FIG. 6, the request 510 is applied to the rootpage 515 which provides the page ID 516 for the next lower node in thedistributed consistent database's hierarchy that is appropriate tosatisfy the request 603. The next lower page ID 516, in turn, isprovided to the mapping table 512 which provides 604 the object ID 517for the object 518 in KVS 501 that contains the next lower page 519. Theobject 518 containing the next lower page 519 is fetched 605 from KVS501 and the process repeats 606 until a leaf page is reached 607 in thedistributed consistent database's hierarchy.

For ease of drawing FIG. 5d depicts a leaf page 520 being reachedimmediately after the first intermediate page 519. Note that only threepages have been called up from KVS to reach the leaf page 520 whereasthe distributed consistent database's hierarchy of pages may be muchlarger than that (e.g., many more intermediate pages and leaf pages mayexist).

In an embodiment, object ID(s) for requested information are founddirectly within a leaf page (i.e., the mapping table is not used). Forexample, in the case of directory request “/C/RED/BLUE/FILE”, the objectID for FILE will be found in the leaf page that is called up from KVS ofthe distributed consistent database dedicated to the folder BLUE. Theobject ID is then applied directly to KVS to fetch the requestedinformation 608. Here, the object ID for the head object for thedistributed consistent database representing folder BLUE is referencedon a leaf page of a distributed consistent database representing folderRED. The object ID for the head object for the distributed consistentdatabase representing folder RED will be referenced on a leaf page of adistributed consistent database representing root directory C. In animplementation, a distributed consistent database representing a rootdirectory has a fixed object ID.

Although the embodiments described herein mainly contemplate animplementation where only one page is kept in an object containing ahierarchical page, it is conceivable that multiple hierarchical pagesmay be kept in single object.

3.3 The Distributed Consistent Database—Implementation of Changes

FIGS. 7a-7e show distributed consistent database behavior when changesare made to the distributed consistent database. FIG. 8 shows amethodology outlining such behavior.

The contents of a distributed consistent database are often not static.The hierarchy of a distributed consistent database can behave much likea B− tree in the face of changes. For instance, if a large number ofobjects is added to the distributed consistent database for storage, thenumber of intermediate and/or leaf pages in the hierarchy may expandoutward. Likewise, if a number of objects kept by the distributedconsistent database are deleted, content of multiple pages may be mergedonto a single page with one or more of the other pages being deleted.Thus, the “shape and size” of a distributed consistent database's pagehierarchy may expand and contract in response to the addition anddeletion of objects that are stored in the distributed consistentdatabase. Moreover, changes to pages should have limited ripple effectsto other pages. For instance, if a leaf page is deleted, the deletionneeds to be accounted for in the intermediate page that references thedeleted leaf page but no other page needs to be modified.

A practical difficulty is that, as discussed above with respect to FIG.2, the overall framework is intended to have the capability to scale tolarge values of N (number of DDS instances) and/or M (number of users).As such, it is possible that one or more other DDS instances and/orusers may be simultaneously accessing the same distributed consistentdatabase. In an implementation, there is no limit placed on how manyreads may be made from KVS for the head object for a particulardistributed consistent database. As such, anytime a particular user orDDS instance intends to impose a change to a distributed consistentdatabase there is the potential that a large number of other DDSinstances and/or users are using the distributed consistent database andintend to make their own changes as well. Some of these changes may evencompete with the changes desired by the particular user or DDS instance.

In an implementation this problem is dealt with by making the objects ofa distributed consistent database other than its head object immutable(that is, they cannot be modified), and, using an ACID transactionalprocess on KVS to effect any desired changes to the distributedconsistent database. Here, in embodiment, objects containingintermediate pages, objects containing leaf pages, and the customer dataobjects referred to by the leaf pages are all made immutable.

As observed in FIGS. 7a and 8, in an embodiment, when the head object711 for a structure is first read 801 from KVS a “working” mapping table720 is created from the mapping table 712 contained within the headobject 711. Initially the working mapping table 720 is just a copy ofthe mapping table 712 that is found in the head object 711. In animplementation, the head object 711 also contains meta-data 725 thattracks versioning for the head object 711. The use of the workingmapping table 720 and the version number for the head object 711 willbecome more apparent in the following discussion.

The distributed consistent database is then accessed consistent with themethodology of FIG. 6 and the specifics of the request. The access mayentail deletions of and/or additions to the customer data objects keptby the distributed consistent database. Such changes may cause changesto the content of the intermediate and/or leaf pages of the distributedconsistent database's hierarchy, and/or, may change the number ofintermediate and/or leaf pages. In the case of customer objects to bedeleted, one or more leaf pages will have to be amended to delete anyreference to their object IDs. In the case of newly created customerobjects, certain leaf pages will have to be amended (and/or added) toinclude references to their object IDs. The specific pages that areimpacted by any such changes are identified and called up from KVS 802(if not already called up by way of the prior access). For ease ofdrawing, FIGS. 7a-7e do not show any of the hierarchy pages.

As observed in FIG. 8, there are three basic kinds of changes that canbe made to the pages that are impacted by the structural change: 1) thedeletion of a page 803; 2) the addition of page 804; and, 3) themodification of a page 805. Any number and combination of these changesmay result from any combination of additions and deletions to the set ofcustomer objects kept by the distributed consistent database.

Referring to FIG. 7b and FIG. 8, if a page is to be deleted 803, itsentry 721 is stricken 806 from the working mapping table 720. If a pageis to be added 804, a new entry 722 having a new page ID andcorresponding object ID for the new page is added 807 to the workingmapping table 720. If a page is to be modified 805, the change is madeto the page (which can be a change, deletion and/or addition upon thepage's information), a new object ID 723 for the page is created andentered to the working mapping table 720 (the modified page keeps itspage ID), and, the old object ID 724 for the page is stricken 808 fromthe working mapping table 720 (the old object containing the old pagewill be deleted from KVS). Note that process 808 essentially accountsfor the characteristic that KVS objects containing pages are immutable.Here, permitting a modified page to keep its page ID dampens the “rippleup” of changes upward in the page hierarchy that would otherwise need tobe made if a new page ID were used.

The head object 711, however, is not immutable and the DDS instance,referring to FIGS. 7c and 8, initiates a PUT operation to KVS toessentially incorporate the updated working mapping table 720 into thehead object 711 rather than the original mapping table 712 that wasfound in the head object 711 when it was first read from KVS (at process801). Recall that in the case of a PUT operation, a RESERVE command isinitially sent 809 to every KVS node that stores the object or a replicaof the object. A subsequent PUT command 813 is not issued to any suchnode until an appropriate response is received from each node to which aRESERVE command was sent 810.

In the case of a PUT operation, an appropriate response to a RESERVEcommand 809 includes the object's meta data with current version number726, and, an indication that the specific object exists and there are nocompeting PUT, GET or DELETE requests for the same object. Receipt ofthe appropriate response from all KVS nodes having the head object orits replica confirms that the RESERVE command has locked the headobject.

The current meta-data version number 726 for the head object 711 that isreceived in response to the RESERVE command is compared 811 with theversion number 725 for the head object 711 when the head object 711 wasfirst read from KVS 801. If the version number has not changed since theinitial reading 801 of the head object 711 (i.e., the current versionnumber 726 for the head object included in the RESERVE responses is thesame as the version number 725 for the head object as of the initialread of the head object 801) there were no changes made to thedistributed consistent database since the initial access 801 of the headobject and the changes are committed 812.

Referring to 7 d and FIG. 8, the changes are committed 812 by: 1)issuing one or more CREATE commands 813 into KVS for one or more newobjects for any new pages, modified existing pages, new customer objectsor effectively modified pre-existing customer objects; 2) issuing a PUTcommand 814 to KVS for the head object 711 including the working mappingtable 720; 3) issuing one or more DELETE commands 815 for one or moreobjects to be deleted corresponding to deleted pages, earlier versionsof modified pages or earlier versions of modified customer data objects.In an embodiment, at least processes 813 and 814 are executed seriallyin order.

As alluded to above, the one or more CREATE commands store data objectsinto KVS for any new customer data objects to be added to the group ofobjects 403 kept by the distributed consistent database. These includeboth entirely new items of data and modified versions of previouslyexisting data (the later reflecting the immutable nature of the customerdata objects). The object IDs for the newly created objects may berandomly chosen. The one or more DELETE commands are also used to deletefrom KVS any customer data objects to deleted from the group of objectskept by the distributed consistent database. These include bothpreviously existing items of data that have been deleted outright andpreviously existing items of data that have been modified (the lateragain reflecting the immutable nature of the customer data objects).

Referring to FIG. 7e and FIG. 8, if the version number has changed, thechanges to be made to the distributed consistent database's hierarchyare rolled back 816 by: 1) not following through with a PUT command onthe head object (as such, the version number for the head object is notincremented and the original mapping table is maintained); 2) discardingthe working mapping table 720; and, 3) discarding any newly createdpages (which effectively discards modified as well as actually newpages).

Note that the use of the RESERVE command for PUT operations correspondsto an optimistic locking system when the potential for multipleconcurrently existing instances of the same DDS structure is considered.The number of such instances can be very large.

It is important to point out that although the KVS solution describedabove in Section 2.0 was presented as the underlying KVS in thediscussion of the distributed consistent database provided justdescribed above in Sections 3.0, 3.1, 3.2 and 3.3, such a presentationwas only for exemplary purposes. Other embodiments that implementdistributed consistent database technology may use KVS technologiesother than the particular KVS solution described above in Section 2.0.Here, at least, any KVS system that implements reservations andversioning akin to processes 809-811 of FIG. 8 may be used to implementdistributed consistent database technology (safety and liveness may beadditionally guaranteed).

3.4 Uses of the Distributed Consistent Database to Implement Large ScaleStorage Systems

The generic access system discussed above has a number of different usesfor effectively extending the range of KVS beyond a basic object store.These include directory and thin provisioned file storage systems. Adiscussion of each of these is provided immediately below.

i) Directory File Storage Systems

FIG. 9 depicts an exemplary use of the distributed consistent databaseto implement the file directory interface 206 of FIG. 2. As is known inthe art, a specific file is targeted in a file directory system byspecifying a pathway of sub-directories/folders through the directory'shierarchy (e.g., “/RED/BLUE/GOLD/FILE”). FIG. 9 depicts a segment 930 ofa file directory's hierarchy. Here, a single sub-directory/folder 931 isshown. The sub-directory's parent directory 932 is shown as is itschildren sub-directories and files 933_1 through 933_R. A “root node”head object represents the entire file system directory and paths to allsub-directories and files flow from the root node.

In an implementation, every “node” in the file system in implemented asa distributed consistent database having its own head object, etc. Thus,parent directory 932 is implemented as a first distributed consistentdatabase instance, sub-directory 931 is implemented as a seconddistributed consistent database instance and each of the childrensub-directories and files 933_1 through 933_R are implemented as theirown individual distributed consistent database instances.

FIG. 9 shows a representation 934 of the distributed consistent databaseused to implement sub-directory 931. Consistent with the discussionsprovided above, the distributed consistent database includes a hierarchyof pages 935 and a group of objects 936 that are kept by the distributedconsistent database. For any given access to the distributed consistentdatabase, the hierarchy of pages 935 are navigated through in piece-mealfashion to reach a specific one or more objects from the group ofobjects 936 kept by the distributed consistent database.

Here, the group of objects 936 kept by the distributed consistentdatabase correspond to the children sub-directories and files 933_1through 933_R within the sub-directory 931. Each of these objects 936are also implemented as head object (each child sub-directory and fileis implemented as its own distributed consistent database). Thehierarchy of pages 934 permits for easy/efficient access of any targetedchild sub-directory or file should R become extremely large.

A targeted file is reached anywhere in a file system directory by“hopping” through a chain of distributed consistent databasesrepresenting the directory/sub-directory path of the request until thedesired “file” is reached (which is also implemented as a distributedconsistent database). The content of the hierarchy of pages is designedto guide the navigation progression through the pages consistent withthe request which specifies a file directory path.

For example, for any access that “flows” through sub-directory 931, thehead object for the parent directory 932 will be called up from KVS, itshierarchy of pages will be navigated through piece-meal until a leafpage that identifies the head object for sub-directory 931 is reached.The head object for sub-directory 931 will then be called up from KVSand its hierarchy of pages 935 will be navigated through until a leafpage is reached that identifies the head object of the nextsub-directory that is impacted by the request. The process continuesuntil the targeted file is reached. If the file targeted by the requestis contained by sub-directory 931 a leaf page will be reached inhierarchy 935 that identifies the head object for the targeted file.

Changes to the file system are made consistently with the discussionabove in FIGS. 7a-7e and FIG. 8.

If any files or sub-directories 933_1 through 933_R within sub-directory931 are to be deleted, their corresponding head objects are marked fordeletion from KVS and the hierarchy of pages 935 are modified to nolonger refer to the object IDs of these sub-directories/files. Thehierarchy of pages 935 are modified at least by removing the object IDsof the deleted sub-directories/files from any leaf pages of thehierarchy 935. This may include effective page modification, entire pagedeletion or both.

For example, deletion of a large number of sub-directory/file objectsmay cause two leaf pages to merge. This causes modification to the leafpage that takes on the content of the leaf page to be deleted, and,causes modification to the intermediate page that used to refer to bothleaf pages (and now needs to only refer to the one remaining leaf page).No other page modification is required. The fact that modified pageskeep their page ID dampens ripple effects of page modification extendingupward in the hierarchy.

Owing to the immutable property, modified pages keep their page ID butreceive a new object ID in the working mapping table and are marked forstorage in a new object in KVS. Objects containing the older content ofa modified page are marked for deletion. Pages that are deleted outrighthave their entries in the working mapping table deleted and have theirKVS objects marked for deletion.

The head object for sub-directory 931, which is not immutable, is thenwritten into KVS with the new working mapping table. Head objects ofsub-directories/files to be deleted are deleted from KVS. New objectsfor freshly modified pages are created into KVS and objects containingold modified pages or entirely deleted pages are deleted from KVS.

If any files or sub-directories are to be added to sub-directory 931,new head objects for each are marked for creation in KVS, and thehierarchy of pages 935 are modified to reference these new files orsub-directories. The hierarchy of pages 935 are modified at least byadding references for the object IDs of the newly addedsub-directories/files to one or more leaf pages of the hierarchy 935.This may include effective page modification, entire page addition orboth. For example, addition of a large number of sub-directory/fileobjects may cause a single leaf page to split into two leaf pages. Thiscauses modification to the existing leaf page (which takes on referencesto new object IDs but deletes some references to pre-existing objectIDs), addition of a new leaf page, and modification to the intermediatepage that used to refer to the single leaf page but now has to refer toboth.

As mentioned above, owing to the immutable property, modified pages keeptheir page ID but receive a new object ID in the working mapping tableand are marked for storage in a new object in KVS. Objects containingthe older content of a modified page are marked for deletion. Pages thatare added outright have new entries created in the working mapping tablefor them and are marked to have new objects created for them in KVS.

The head object for sub-directory 931, which is not immutable, is thenwritten into KVS with the new working mapping table. Head objects ofsub-directories/files being added are created into KVS. New objects forfreshly modified pages or new pages are created into KVS and objectscontaining old modified pages are deleted from KVS.

In an embodiment, each page ID of sub-directory 931 and eachsub-directory ID and file ID kept by distributed consistent database isimplemented as an “inode” number. The key into KVS to fetch the objectfor any of these items is obtained by concatenating the inode numberwith an identifier of the overall file system.

In an embodiment each file is implemented as a distributed consistentdatabase. As such, the content of any file can be large yet be easilyaccessed and/or manipulated through the characteristics of thedistributed consistent database.

ii) Thin Provisioning Block Storage Systems

Recall from the background discussion of FIG. 1c that a common type ofblock storage is a “thin provisioning” system in which a block or“stripe” is identified by an offset within a larger “logical” storageresource, and where, “used” stripes are written into physical storagebut unused stripes are not.

FIG. 10 shows a “thin provisioned” block device implemented as a single“sparse file” that is represented as a distributed consistent database1001. Here, the leaf pages 1002 of the distributed consistent database'shierarchy 1003 contain mappings that correlate an offset identifying aparticular stripe of the sparse file to a particular object ID (that is,each object in KVS 1004 corresponds to a different stripe). In anembodiment there is one such mapping per leaf page so that, e.g., anytwo different stripes within a same sparse file can be simultaneouslyaccessed. Other embodiments may choose to map multiple stripe objectsfrom a single leaf page. Basic accesses to the distributed consistentdatabase 1001 specify the offset of a particular stripe. The root andintermediate pages of the hierarchy of pages 1003 point to a correctlower page based on the offset specified in the access request. That is,the pages within the hierarchy 1003 of the distributed consistentdatabase contain content to navigate to a particular leaf page based onan offset as the specified input criteria.

The content of the sparse file's stripes are stored in KVS objects. Noeffort is made to store the full extent of the unused portions of thelarger file in KVS.

The behavior of a distributed consistent database that implements asparse file will be as described above with respect to sections 3.1, 3.2and 3.3. Here, in an embodiment, objects other than the head object usedto implement a sparse file are made immutable (whether corresponding tointermediate or leaf pages or objects containing data stripes).

In one embodiment, all stripes are configured to be the same size.However in other embodiments, given that each stripe is implemented withits own KVS object, different stripes within a same thin provisionedfile implementation can have their own custom/unique size configuration.

In order to increase throughput and performance, in an embodiment, onlywhole stripes are written to the KVS layer unless the application layerissues a flush on an incomplete stripe (in this case only the relevantportion of the stripe is written to KVS). A typical situation is thecopying of an entire file in which copies are made in smaller sectionsthan the full stripe size. For example, if a stripe is 1.6 Mb in size,sections of the copy for storage in the stripe may be made by the userin 16 Kb increments. In this case, the stripe is not written to until1,000 such sections have been aggregated to form a full sized stripe'sworth of data. The aggregation may be made anywhere above the KVSinterface (e.g., by the DDS instance, or locally at the user's system).As discussed above, a KVS object containing a stripe is immutable so anysuch “write” is effected by writing the new stripe's worth ofinformation into an entirely new object.

Operations to a file within a directory file storage system may besimilarly aggregated (e.g., smaller sections of the file are aggregatedabove the KVS layer, then, a larger write operation is made to KVS thatcorresponds to the accumulation of all the aggregated parts).

Sparse files can be implemented as the “files” in a directory filesystem (e.g., as described above with respect to FIG. 9), or, some-otheraccess structure may be used to access a particular one of many thinprovisioned files.

3.5 Uses of the Distributed Consistent Database to ImplementApplications on Top of Storage Systems

Section 3.2 discussed the implementation of the file system directoryand block storage system interfaces 206, 207 of FIG. 2. FIG. 2 alsoindicates that higher level applications such as quotas, and relationaldatabase layers can also be implemented with distributed consistentdatabases in KVS.

i) Quotas

A quota policy is typically used to assign a maximum amount of usablestorage space to a particular user. In a common application, anytime auser attempts to write information into KVS, the quota policy is firstaccessed to understand what the user's maximum allotted storage amountis and how much of that allotment the user has already consumed. If theuser's attempted write will exceed the user's allotted amount the user'swrite attempt is blocked.

In an implementation, a quota policy is implemented with a singledistributed consistent database as discussed above. Here, an identifierof the user is used to first navigate through the node hierarchy of thequota distributed consistent database before the user's access ispermitted. The object ID for an object in KVS having the quotainformation for a particular user is reached when a leaf page of thedistributed consistent database's node hierarchy is reached. If theuser's quota is will not be exceeded by the user's desired access, theaccess for the user is permitted into another database where the user'sinformation is kept.

Additions, deletions and changes of user identifiers will cause changesto the distributed consistent database which may include changes to oneor more pages in the node hierarchy of the distributed consistentdatabase. The behavior of each affected distributed consistent databasewill be similar to that described above in Sections 3.1 through 3.3.

FIG. 11 shows a basic quota process. As observed in FIG. 11, an accessfor a user to (e.g., a distributed consistent) database whereinformation for the user is kept is desired 1101. Instead of accessingthis database, however, another distributed consistent database isfetched from KVS that keeps quota policies for the database 1102. Someform of identifier of the user is then applied to the quota database andits pages are progressed through as discussed above until a leaf pagehaving an object ID for the user's quota record is obtained 1103. Theobject having the user's quota record is fetched from KVS. The quotarecord keeps information indicating how much storage space the user hasused and how much storage space the user has been allocated. Bycomparing the contents of the record against details of the pendingaccess, a determination can be made if the user will exceed the user'squota. If the pending access does not exceed the quota the user's accessto the database is permitted 1104, if the pending access exceeds thequota the user's access is not permitted 1105.

Note that quota information is not limited to tracking capacity. Thequota mechanism can be leveraged for other metrics, like the number offiles for example.

ii) Relational Databases

Distributed consistent databases can also be used to implement arelational database. As observed in FIG. 12, the basic structures for arelational database include a first distributed consistent database 1201that is used to implement a specific table (“table database”). Here,each object 1202 kept by the table database 1201 corresponds to adifferent row in a table of the relational database (“row object”). Therow objects are retrieved with specific KVS object IDs. The hierarchy ofpages 1203 of the table database 1201 accept primary key information fora particular row as input information. The resultant leaf page for theprimary key input provides the object ID for the particular, targetedrow object.

Queries into the table can be sped-up with one or more indexes 1204_1through 1204_N. In an embodiment, each of the indexes 1204_1 through1204_N is also implemented with its own respective consistentdistributed database (“index database”), where, the objects “kept” by anindex database correspond to the head objects of the table databasesthat the leaf pages of the index database point to. Here, the respectivehierarchy of pages 1204_1 through 1204_N of each index databaseeffectively implements the B+ tree of the index that the index databaserepresents. The leaf pages provide the primary keys of the rows thatmatch the query term. In a basic approach, there is one index for eachcolumn in the row structure (e.g., one index for every separate dataitem kept in the row objects 1202 of the table database 1201).

The structure of the primary key that is provided by an index includesan identifier of the head object for the table database (or informationfrom which the head object for the table database can be determined).For example, the primary key may include both a table identifier and arow identifier. The table identifier is used to fetch the head objectfor the table database 1201 and the row identifier (or both the tableidentifier and the row identifier) are applied to the hierarchy of pages1203 of the table database to obtain the object ID for the rowidentified by the primary key. Notably hashing may be performed on anyof the components of a primary key provided by an index to obtain eitheror both of the object ID for the head object of the table databaseand/or the row identifier.

Other table databases for other tables within the relational databasemay be implemented with other consistent distributed databases (notshown in FIG. 12 for simplicity). The row object of one table databasemay include primary key information for another row object of anothertable database consistent with the operation of a relational database.As such, for example, a query process may entail firstly application ofa query term to an index database 1204_1 which provides the primary keyfor a row object within a first table database. The contents of the rowobject may include a primary key for the row of a second table database.This primary key may then be used to access the second table database(through its head object, the primary key including or being useable toobtain the head object for the second table database) and the row withinit specified by the primary key to obtain the result of the query.

In a simplest case all of the primary keys from an index databaseidentify the same head object for a single table database althoughconceivably a single index database may provide primary keys fordifferent table databases and their corresponding head objects. Any oneor more of the indexes may be designed to support range queries orvarious other properties of an RDBMS.

Another “root” head object (not shown) may represent the entirerelational database by including at least identifiers of the variousindex databases used to access the table databases. Thus, access to aparticular relational database causes its corresponding root head objectto be obtained. The query is then applied to the appropriate indexdatabase identified therein.

Changes to table databases in terms of addition or removal or rows (rowobjects) causes corresponding addition or removal of information withinthe content of the pages of the indexes that reference the added/deletedrows.

Quota systems can be applied to relational databases. For example a usermay be limited on how many row objects the user is permitted to enterwithin the relational database. ACID transactions can also be performedon the relational database.

4.0 Implementation Embodiments

Any combination of the storage services/technologies discussed in thepreceding sections may be made to provide storage services for one ormore users.

FIG. 13 is a schematic depiction of various manners in which thearchitecture of FIG. 2 can actually be implemented in practice. Asobserved in FIG. 13, multiple CNS instances 1303_1 through 1303_Nincluding their corresponding DDS stack 1302_1 through 1302_N can beconfigured to run on their own operating system instance and virtualmachine 1350_1 through 1350_N. A single computing system (e.g., aserver, a personal computer, a tablet device, a mobile handheld device,etc.) may support the execution of one or more CNS instances. In thecase where multiple CNS instances are executed within a single computingsystem, the respective virtual machines of the multiple CNS instancesmay run on a virtual machine monitor. A CNS instance may also run on anoperating system that runs directly on a computing system's CPU hardware(i.e., no virtual machine is utilized).

Regardless, as depicted in FIG. 13, there may be more than one computingsystem 1351_1 through 1351_N each having at least one CNS instance withcorresponding DDS stack.

As discussed with respect to FIG. 2, each CNS instance may supportmultiple users 1305_1 through 1305_N. The multiple users may beseparated from their respective CNS node(s) by any of a wide areanetwork (WAN), metropolitan area network (MAN), local area network(LAN). A user may even be an application instance running on the samecomputing system that is supporting its CNS node (no external network).Multiple users may be connected to any or all of the CNS nodes by way ofa same network or multiple networks.

Likewise, each of the CNS nodes may be connected to KVS through any of aWAN, MAN or LAN. Conceivably a CNS node may run on a same computingsystem upon which a KVS node is instantiated (i.e., no external networkbetween the CNS node and the KVS node). Multiple CNS instances may beconnected to KVS by way of a same network or multiple networks.

KVS, as discussed above in Section 2.0, may be implemented as adistributed storage system. In this case, the various distributed nodesof the KVS system are typically separated by a MAN, but may also beconceivably separated by a LAN and/or WAN.

A “cloud storage” service may be provided where the CNS nodes acts asgateways to the cloud storage service.

Processes taught by the discussion above may be performed with programcode such as machine-executable instructions which cause a machine (suchas a “virtual machine”, a general-purpose CPU processor disposed on asemiconductor chip or special-purpose processor disposed on asemiconductor chip) to perform certain functions. Alternatively, thesefunctions may be performed by specific hardware components that containhardwired logic for performing the functions, or by any combination ofprogrammed computer components and custom hardware components.

A storage medium may be used to store program code. A storage mediumthat stores program code may be embodied as, but is not limited to, oneor more memories (e.g., one or more flash memories, random accessmemories (static, dynamic or other)), optical disks, CD-ROMs, DVD ROMs,EPROMs, EEPROMs, magnetic or optical cards or other type ofmachine-readable media suitable for storing electronic instructions.Program code may also be downloaded from a remote computer (e.g., aserver) to a requesting computer (e.g., a client) by way of data signalsembodied in a propagation medium (e.g., via a communication link (e.g.,a network connection)).

FIG. 14 is a block diagram of a computing system 1400 that can executeprogram code stored by a storage medium. It is important to recognizethat the computing system block diagram of FIG. 14 is just one ofvarious computing system architectures. Different types of computingsystems include mobile and/or handheld computing devices (e.g.,smartphones, cell-phones, personal digital assistances), laptop personalcomputers, desktop personal computers, servers, etc.

The applicable storage medium may include one or more fixed components(such as non volatile storage component 1402 (e.g., a hard disk drive,FLASH drive or non volatile memory) or system memory 1405) and/orvarious movable components such as a CD ROM 1403, a compact disc, amagnetic tape, etc. operable with removable media drive 1404. In orderto execute the program code, typically instructions of the program codeare loaded into the Random Access Memory (RAM) system memory 1405; and,the processing core 1406 then executes the instructions. The processingcore 1406 may include one or more CPU processors or CPU processingcores.

It is believed that processes taught by the discussion above can bedescribed within various source code software environments such as, forexample, object-oriented and/or non-object-oriented programmingenvironments including but not limited to: C+/C++, PYTHON, Java, Erlang,JavaScript, etc. The source code can be subsequently compiled intointermediate code for translation on a translator/virtual machine, or,compiled into object code targeted for a specific processor instructionset architecture.

In the foregoing specification, the invention has been described withreference to specific exemplary embodiments thereof. It will, however,be evident that various modifications and changes may be made theretowithout departing from the broader spirit and scope of the invention asset forth in the appended claims. The specification and drawings are,accordingly, to be regarded in an illustrative rather than a restrictivesense.

5.0 Snapshots of Storage Systems

Storage systems typically change over time as they are used. That is,the ability to change existing data items, add new data items, addfolders, delete folders, etc., causes the organization and content ofthe storage system to continually change as it is continually used.Various users or administrators may, however, desire to retain the stateof a storage system at a particular moment in time (e.g., for laterreference) yet permit changes to continue to be made to the systemmoving forward. A “snapshot” is the state of storage system (or storagesystem component) at a particular moment in its usage that is kept forsubsequent referral. Typically, the storage system can continue to beused and changed after the snapshot is taken.

FIG. 15 shows a depiction of a system that is more advanced than theinitial system originally depicted in FIG. 2. Comparing FIG. 15 withFIG. 2, note that the more advanced system of FIG. 15 includes theincorporation of snapshot technology 1515_1 through 1515_N associatedwith the distributed consistent database 1513_1 through 1513_N and DDScomponents 1502_1 through 1502_N of each CNS node 1503_1 through 1503_N.The snapshot components 1515_1 through 1515_N of the CNS nodes 1503_1through 1503_N of FIG. 15 implement snapshot methodologies consistentthe discussions provided immediately below so that various forms ofsnapshots may be taken of any of the large scale distributed storagesystems described in the preceding sections.

FIG. 16 pertains to an approach for taking snapshots of a storage systemimplemented with distributed consistent databases as discussed inprevious sections (whether the storage system is implemented as anobject store, a file system, a block storage system or a relationaldatabase any of which may or may not use quotas). As the KVS layer iscapable of simultaneously keeping the content of multiple, differentstorage systems, it is understood that although the methodologies ofFIG. 16 are performed on a single system as an example, suchmethodologies may be concurrently performed on multiple ones of thedifferent storage systems implemented in KVS.

As observed in FIG. 16, over an initial time period 1601, the storagesystem 1602 is used and continually changing. Before any snapshots aretaken of the system, a snapshot counter is given an initial value thatindicates no snapshots exist (e.g., snapshot counter=0). Here, sequence1603 depicts changes being made to a particular distributed consistentdatabase 1604 over the time period 1601, and, sequence 1605 depictschanges being made to another particular consistent database 1606 overthe time period 1601. Consistent with the discussions of previoussections, the storage system 1602 is composed of multiple distributedconsistent databases of which database 1604 and database 1606 are part.

At moment 1607 a snapshot of the entire storage system 1602 is taken. Inan embodiment, the taking of a snapshot principally involvesincrementing the snapshot counter and understanding that certain metadata and actions are to be taken with respect to the storage system'sdistributed consistent databases and user data.

After the snapshot is taken the database continues to be used over timeperiod 1608 which causes additional changes to be made to the database.A second snapshot is taken at moment 1609 and a third snapshot is takenat moment 1610. As can be observed in FIG. 16, changes continue to bemade to distributed consistent database 1604 within time periods 1608,1611 and 1613 after their respective snapshots 1607, 1609, 1610.Moreover, changes are made to distributed consistent database 1606 inboth time periods 1611 and 1613 but changes are not made to distributedconsistent database 1606 in time period 1608.

When the first change 1612 is to be made to distributed consistentdatabase 1604 after the first snapshot 1607 is taken certain actions aretaken to effectuate the first snapshot.

As observed in FIG. 16, the “active/primary” version of database 1604has an associated “current tag” value which corresponds to the value ofthe snapshot counter the last time a snapshot was taken of consistentdatabase 1604. In an embodiment, the head object for every distributedconsistent database within the storage system has its own associatedcurrent tag value. Moreover, at the creation of the storage systemand/or after all snapshots of the storage system have been deleted, thesnapshot counter is reset to a value of 0 and all current tags of allconsistent databases within the storage system are reset to a value of 0(in various embodiments, the counter is set to zero at the creation ofthe file system and subsequently is never reset). New consistentdatabases that are added to a storage system having existing snapshotsare given a current tag value of “null” to indicate they do not belongto any of the snapshots.

Here, commensurate with changes 1612 being made to active/primarydatabase 1604, the value of the current tag of consistent database 1604(0) is compared against the value of the snapshot counter (1). Notably,the current tag of database 1604 is less than the value of the snapshotcounter. A consistent database whose current tag value is less than thevalue of the snapshot counter corresponds to a consistent database whosestate needs to be saved if any changes are to be made to the consistentdatabase. That is, the change 1612 that is about to be made to database104 corresponds to its first change since the most recent snapshot 1607of the system was taken. As such, a number of actions are taken.

First, the state of consistent database 1604 before any of the newchanges 1612 are made are saved into KVS. This saved version of database1604 can be referred to as the “first parent” 1604′ of theprimary/active database 1604. First parent database 1604′ is given acurrent tag value of 0 to indicate that was the current tag value of thefirst parent's content when the snapshot was taken.

Second, the current tag value of the primary/active database 1604 isupdated to be set equal to the current value of the snapshot counter(1). In an embodiment, all subsequent accesses to the primary/activedatabase 1604 will perform the same comparison of the database's currenttag value against the current value of the snapshot counter. Because allsuch comparisons will not yield a difference between these two valuesuntil the next snapshot is taken, changes can continually be made to theprimary/active database 1604 over a series of separate accesses 1614(before the next snapshot 1609 is taken) without triggering any savingof distributed consistent database state to KVS.

In an embodiment, the first parent database 1604′ is realized with thesame head object 1615 having same object ID as the head object ofactive/primary database 1604 prior to changes 1612, and, a new secondhead object 1616 for the version having changes 1612 is created with anew object ID that will be used to implement the active/primary database1604 going forward. In other embodiments an opposite approach can betaken. As will be described in more detail further below, both headobjects 1615, 1616 are obtainable through an “object proxy” (notdepicted in FIG. 16) that a leaf page of, e.g., a higher level folderthat contains database 1604 refers to instead of either of these objects1615, 1616 specifically. As such, changes 1612 can be made toactive/primary database 1604 and the first parent snapshot 1604′ can becreated without any changes having to be made to such a leaf page.Object proxies are discussed in more detail further below.

The head object 1615 of the first parent 1604′ will have the mappingtable 1617 as it existed in the active/primary database 1604 before anyof the changes 1612 were made. By so doing, the state of database 1604as it existed just prior to changes 1612 is preserved. If changes 1612modify one or more pages of the hierarchy of pages of primary/activedatabase 1604, new object IDs for affected pages would be listed in anew, updated mapping table 1618 kept within the new primary/activedatabase head object 1616. As a consequence of the immutable property,the original pages will still be preserved in KVS and will be referredto by the (older) mapping table 1617 in the head object 1615 of firstparent 1604′.

The actual implementation of the first parent database 1604′ from headobject 1615 therefore includes sharing of pages and data items that arenot modified by the changes 1612. For example, as discussed just above,changes 1612 may remove a few pages from the database's hierarchy ofpages and replace them with new ones. All other (unchanged) pages arecontained in objects that are referred to by both the updated mappingtable 1618 of the new primary/active database head object 1616 and theolder mapping table 1617 of the first parent 1604′ head object 1615.These principles remain in force for all subsequent changes made priorto the next snapshot 1609.

If active/primary database 1604 corresponds to a folder in a file systemor block storage implementation and a change is made to a lowerdistributed consistent database that is referred to by a leaf page ofdatabase 1604, the change made to the lower database will be handled nodifferently than changes 1612 made to database 1604. Note that if achange is made to the lower database, the current tag associated withthe head object of the lower head object will also be updated to a valueof 1 (consistent with the discussions above). An object proxy of thelower database (again, objects proxies are discussed in more detailbelow) is used to differentiate which version of the lower database (thesnapshotted version or the version with changes) is to be actuallyfetched from a leaf page of database 1604 during subsequent accesses.

Note that distributed consistent database 1606 has no changes made to itin the time period 1608 between the first 1607 and second 1609snapshots. As such, when the first change 1619 is made to database 1606after the second snapshot 109, the treatment will be as described abovefor database 1604 and changes 1612. An exception is that the current tagof database 1606 will be given a value of 2 (and not 1) to reflect thatthe first parent 1606′ of database 1606 was created after the secondsnapshot 1609 and not the first snapshot 1607. The first parent 1606′will have a current tag of 0 to indicate that it represents a versionthat has not changed since before snapshot 1607 was taken.

Changes 1620 are also made to active/primary database 1604 after thesecond snapshot 1609 which results in the creation of its second parentdatabase 1604″. The second parent 1604″ will have a current tag valueof 1. The current tag of the active/primary database 1604 incrementsfrom 1 to 2 as a consequence of these changes.

After the third snapshot 1610 is taken, changes 1621, 1622 are againmade to both active/primary databases 1604, 1606 which increases thecurrent tag of both to 3 and creates third 1604′″ and second parents1606″ respectively each having current tag values of 2.

FIG. 17 shows an exemplary object proxy 1704 for database 1604 and anexemplary object proxy 1706 for database 1606 after changes 1621 and1622 are made to both databases respectively. As observed in FIG. 17,the object proxy 1704 for database 1604 lists the various versions ofdatabase 1604 including the active/primary version and all its parentsalong with their respective object ID and associated current tag value.Likewise, the object proxy 1706 for database 1606 lists the variousversions of database 1606 including the active/primary version and allits parents along with the object ID of their respective head objectsand associated current tag value.

In an embodiment, each object proxy is implemented as a mutable objector distributed consistent database with the information of the objectproxy contained therein. In an embodiment, the respective leaf page of ahigher level consistent database that “contains” database 1604 or 1606(e.g., a folder that contains the content of database 1604 or 1606) willrefer to the object proxy of a particular database rather than any ofits versions specifically.

The information within the object proxy is used to fetch the correcthead object for the particular version that is targeted by theparticular request. For example, if the access to database 1604 is tomake a change to the current version of database 1604, the object ID forthe head object for the active/primary version of the database will beretrieved from the object proxy and utilized. By contrast, if the accessis for a particular snapshot level of database 1604, the head object forthe correct parent will be retrieved from the object proxy and utilized.

As an example, if a request is made for the snapshot level 2 information(i.e., snapshot 1609) for both databases 1604, 1606 sometime afterchanges 1621 and 1622 are made to the respective databases, therespective object proxies for both databases 1604, 1606 will be fetched.The object ID for the second parent 1604″ (having current tag=1) will beused from object proxy 1704 and the object ID for the first parent 1606′(having current tag=0) will be used from object proxy 1706. Note thatthe correct retrieved version has a tag value that is the closest,lesser value to the desired snapshot level.

If a subsequent request is made for snapshot level 1 information (i.e.,snapshot 1607) for both databases 1604, 1606, again the respectiveobject proxies for both databases will be retrieved. The object ID forthe head object having a current value of 0 will be selected from bothobject proxies (first parent 1604′ and first parent 1606′).

If snapshot level 1 information is no longer desired (the intent is toerase snapshot 1607), again the object IDs for first parents 1604′ and1606′ (having a current tag value of 0) will be retrieved from theobject proxies. Any object IDs of any pages found within the respectivemapping tables of the first parents 1604′ and 1606′ that do not exist onthe respective mapping tables of any later parents or the active/primarydatabases 1604, 1606 can be deleted.

If snapshot level 2 information is no longer desired (the intent is toerase snapshot 1609), the object ID for second parent 1604″ of database1604 will be retrieved from its corresponding object proxy and the sameprocedure discussed just above will be followed. If snapshot level 1 hasnot been deleted as of the deletion of snapshot level 2, the comparisonof mapping table entries will also be made looking backward as well asforward. That is, the mapping table entries of second parent 1604″ willbe compared against first parent 1604′ as well as against its juniorparents and the active/primary version. Again, only the objects forentries that appear on the mapping table of second parent 1604″ but alsodo not appear on the mapping table of first parent 1604′ or any of thejunior snapshots or active/primary database are permitted to be deleted.

The deletion of snapshot level 2 will have no effect on the keptversions of database 1606 regardless if snapshot level 1 was or was notdeleted beforehand. If snapshot level 1 was deleted beforehand, there isno version of database 1606 having a current tag value of 1 to implementdeletion of the snapshot 2 level. If snapshot level 1 was not deletedbeforehand, the (first parent) version 1606′ of database 1606 having acurrent tag value of 0 also preserves the snapshot level 1 informationfor database 1606. Since snapshot level 1 has not been deleted in thiscase, the version having a current tag value of 0 is not permitted to beerased.

The mechanism by which the entries on the mapping table of a seniorparent database are compared against the entries on the mapping tablesof junior parents or the active/primary database may vary fromembodiment to embodiment. According to one approach, the mapping tableof all junior databases (all junior parents and the active/primarydatabase) are retrieved and compared against the mapping table entriesof the parent to be deleted. Any object ID found on the mapping table ofthe snapshot level to be deleted that is not found on any junior mappingtable are marked for deletion and deleted.

According to another embodiment, mapping tables are compared against oneanother when a snapshot is taken. For instance if a second parentsnapshot is taken, its mapping table is compared against the mappingtable of the first parent during the creation and instantiation of thesecond parent snapshot. Any object ID that is found on the mapping tableof the first (senior) parent but that is not found on the mapping tableof the second (junior) parent is understood to be eligible for deletionupon deletion of the first parent snapshot. The mapping table of alljunior snapshots can be compared, e.g., at the time of their creation,to the mapping tables of all their respective senior parent snapshots inthis manner to embed appropriate deletion information into the mappingtables of the senior parents.

Note that care needs to be taken with respect to the operation of aDELETE command performed at the active/primary database. Specifically,the active/primary database is not permitted to delete any objects thatthe mapping table of a currently existing snapshot refers to. As such,in an embodiment, whenever a snapshot of the active/primary database istaken, the entries on the mapping table of the active/primary mappingtable are marked to indicate that the objected IDs listed on the mappingtable are “owned” by the parent snapshot. Any such marked object IDs aretherefore not deleted by the active/primary database even if nominalfunctions would otherwise do so (e.g., change to a hierarchical page).

Any new object IDs that are added to the mapping table of theactive/primary database as a consequence of changes made to theactive/primary database subsequent to the taking of the snapshot are notmarked and are therefore eligible for deletion until the next snapshotis taken at which point all existing object IDs are again marked toprotect them from deletion.

Object IDs that are marked for protection from deletion can neverthelessbe replaced on a mapping table entry. For instance, per normaloperation, if an object containing a page is changed, a new object isused to keep the new page and the entry on the mapping table willcorrelate the ID of the page to the object ID of the new objectcontaining the new page. Thus the old object ID of the old page, if itis protected, will be replaced in the mapping table entry. But becauseit is protected it will not be deleted. In an embodiment, when themapping table entries of an active/primary database are compared againstthe mapping table entries of an earlier/parent, it is understood thatonly the object IDs within active entries are used in the comparison(object IDs that are protected but have been replaced in the mappingtable entries are not used in the comparison).

FIG. 18a-c illustrate some basic storage system snapshot processesdiscussed above. As observed in FIG. 18a , when a change is to be madeto a distributed consistent database, the current tag for thedistributed consistent database is compared to a snapshot counter 1801.If the current tag is equal to the snapshot counter, the change is madeand no further processes are implemented 1802. If the current tag isbehind the snapshot counter, it is understood that a snapshot has beentaken and the distributed consistent database's state as of thatsnapshot have not yet been taken. As such, in response, the content ofthe head object for the distributed consistent database is saved 1803(including its mapping table content), the entries of the mapping tableto be used for the active/primary version of the distributed consistentdatabase going forward are marked to prevent their deletion 1804, andthe current tag value of the active/primary version of the distributedconsistent database is set equal to the snapshot counter 1805. Inconjunction with processes 1803-1805, the change is made to thedistributed consistent database 1802. In an embodiment, the process ofFIG. 18a is carried out for every consistent database in the storagesystem to realize a snapshot of the entire storage system.

As observed in FIG. 18b , when snapshot information for a particulardistributed consistent database is accessed (e.g., as a file or folderwithin a file system or as a file within a thin provisioned blockstorage system), a hierarchy of distributed consistent databases andtheir associated hierarchy of pages are progressed through until a leafpage is reached that references the object ID for an object proxy withinKVS 1811 that represents the distributed consistent database. The objectproxy is fetched 1812 which contains object IDs for the respective headobjects of different snapshot versions of the distributed consistentdatabase (and the active/primary version of the distributed consistentdatabase). A determination is made as to which snapshotted versioncontains the correct information for the desired snapshot level and thehead object for the correct information is retrieved from KVS 1813.

As observed in FIG. 18c , when a decision is made to delete a snapshotlevel, the object proxy for a distributed consistent database isobtained and the head object for the version to be deleted (if oneexists) is obtained 1821. The entries of the mapping table of theretrieved head object are compared against the entries of the mappingtables for all other parent and junior versions of the database as wellas the current/primary version 1822. Entries that exist on the mappingtable of the version to be deleted but do not exist on the otherversions are permitted to be deleted 1823.

In various embodiments, snapshot versions can take on thecharacteristics of an active/primary database. More simply, changes arepermitted to be made to a snapshot and the flow of changes becomes itsown active/primary database.

FIG. 16 indicates that at time 1623 changes are made to first parent1604′ creating version 1604′_1 of database 1604. Changes are then againmade to version 1604′_1 at time 1624 creating version 1604′_2 ofdatabase 1604. When changes are made to a snapshot, a “branch” or “fork”is said to be taken from the active/primary database. That is, thestring of changes 1623, 1624 stemming from snapshot 1604′ and theircorresponding head objects 1604′_1 and 1604′_2 correspond to an entirelyseparate active/primary database level that branched from the originalactive/primary database 1604. In many cases, because a branch beginswith a snapshot and changes can then be made to the snapshot, a “branch”is actually realized as a “read/write” snapshot. Read/write snapshotsare discussed further below. In an embodiment, all object IDs stemmingfrom the mapping table of the original snapshot 1604′ are marked in themapping tables of the branch's subsequent versions 1604′_1, 1604′_2,etc. to prevent the senior objects from being deleted during thesubsequent activity of subsequent versions 1604′_1, 1604′_2. In thismanner the objects referred to by the original snapshot 1604′ arepreserved (and as such snapshot 1604′ is preserved).

In further implementations, snapshots can be taken from branches. Forexample, a snapshot of version 1604′_1 could be taken before the changesthat created version 1604′_2 were made. The snapshot would be processedno differently than the discussion of snapshots provided just above. Itmay be pertinent to point out that a branch typically represents abranch for more than just one distributed consistent database (e.g., abranch for the entire data storage system).

An issue concerns the size of the object proxy when snapshots are takenalong branches. If all versions of an object including snapshot versionsalong branches are kept in the object proxy, the object proxy itself canbecome too large and hamper overall system performance. As such, in anembodiment, the object proxy does not keep object IDs for all of itssnapshot versions.

Instead, each head object of a version of a particular consistentdatabase is configured to refer to its parent. For example, referring toFIG. 16, the head object for the third parent 1604′″ would contain areference to the head object for the second parent 1604″ and the headobject for the second parent 1604″ would contain a reference to the headobject for the first parent 1604′. If the head object for theactive/primary database 1604 keeps a reference to the head object of itsmost recent parent (e.g., the third parent 1604′″ in the example of FIG.16), the object proxy need only identify the head object of theactive/primary database 1604 because all parent snapshots can beobtained by chaining back through the snapshot references (e.g., thehead object for the second parent 1604″ can be obtained by chaining backfrom the active/primary 1604 to the third parent 1604′″, and then fromthe third parent 1604′″ to the second parent 1604″).

If snapshots are being taken from a branch, the object proxyadditionally keeps the object ID for the “head” of the branch. Forexample, again referring to FIG. 16, the object proxy for consistentdatabase 1604 would not only identify the head object for theactive/primary database 1604 but would also identify the head object forthe active/working version 1604′_2 of the branch that stems from firstparent 1604′. The active/working version 1604′_2 of the branch wouldcontain a reference to its snapshot parent 1604′_1 which in turn wouldcontain a reference to its parent 1604′. With this arrangement, theobject proxy essentially only contains the heads of all its branches andprevious snapshot versions along any particular branch can be obtainedby chaining backwards from the heads.

In various embodiments, branches may be taken from branches. Again thesame procedures discussed above may be applied with the only differencebeing that the original snapshot for a “second” branch is taken from abranch and not the primary/active database. The object proxy would bemodified to include the head of the second branch as well.

In an alternate approach, the “head” of a branch is implemented as anobject proxy that keeps the object ID for the working version of thebranch and the object IDs of the head objects for all of the snapshotstaken along the branch and its initial seed version.

In an embodiment, a data structure is kept at the root of the entirestorage system that records which snapshot level each branch was takenon and the parent for each branch. Through this structure the system canfetch the appropriate information for any particular snapshot levelalong any particular branch. The snapshot counter may be “global” witheach snapshot level assigned to any one or more of the primary/activestorage system or any of its existing branches. Here, each time thesystem attempts to access the active/primary storage system or one ofits branches, the system can comprehend what snapshot levels havepreviously applied to the branch being accessed and can implementappropriate procedures accordingly (if certain snapshot levels have notbeen assigned to a branch it can ignore them and interpret current tagsaccordingly). Alternatively each branch may have its own snapshotcounter (where the head of each branch is defined from its parent branchand the snapshot level of its parent branch from where the branchbegins). In an implementation, snapshots levels are regularly removedand the data structure does not record erased snapshot levels whichkeeps the size of the data structure manageable.

Regardless as to how the snapshot information is managed, in anembodiment, snapshots are declared or otherwise associated with aproperty that indicates whether the snapshot is a “read only” snapshot,or, a “read-write” snapshot. If a snapshot is a read only snapshot,changes are not be permitted to be made to the snapshot's recordedinformation of the storage system's state. By contrast, if a snapshot isa read-write snapshot, changes are permitted to be made to thesnapshot's recorded information of the storage system's state. Here, the“branch” that stemmed from first parent 1604′ of FIG. 16 as aconsequence of the series of changes made from the first parent 1604′correspond to a form of a read-write snapshot.

Both read-only and read-write snapshots are easily scaled outward forconcurrent access by multiple users (e.g., across a wide geographicdisperse across multiple CNS nodes). In the case of a read-onlysnapshot, multiple read copies of any/all snapshot information (e.g.,such a particular data object) are freely given as changes can not bemade to the provided information. Even in the case of read-writesnapshots, through the use of head object version numbering (to bedistinguished from current tag values) as discussed above with respectto FIG. 8, conflicts between competing writes to a same consistentdatabase are readily resolved.

FIG. 16 also indicates that in the case of read-write snapshots,multiple branches can be taken from a same snapshot level which permitsease of scaling for multiple users (or sets of users) that wish to writetheir own unique set of changes starting from a particular snapshotlevel in isolation of each other. That is, for example, a first group ofusers may wish to make a series of changes to a particular snapshotlevel in a manner that does not provide visibility to or from changesmade by other users to the same snapshot level.

Here, recalling that sequence of changes 1623 and 1624 (and resultinghead objects 1604′_1 and 1604′_2) correspond to a sequence of changesmade to the first snapshot level of database 1604, consider additionallythat these changes were made by a first group of users that seek tooperate in isolation from other users. Here, a second branch andcorresponding sequence of changes 1633 and 1634 (and resulting headobjects 1604′_11 and 1604′_12) may be made by another group of usersthat also operate from the first snapshot level and seek to operate inisolation of other users (the second sequence of changes 1633 and 1634are understood to operate according to the same principles as the firstsequence of changes 1623 and 1624).

By providing the first group of users the object IDs for head objectsassociated with the first branch (i.e., the object IDs for head objects1604′_1 and 1604′_2) and the second group of users the object IDs forhead objects associated with the second branch (i.e., the object IDs forhead objects 1604′_11 and 1604′_21), each group of users will bepermitted to operate from the first snapshot level of database 1604without visibility into or interference from the changes made by theother users.

Recall that the snapshot approaches described herein efficiently causejunior versions to refer to the unchanged objects of earlier versions.As such, a natural “de-duplication” characteristic can be observed toexist. For example, the mapping table of version 1604′_1 may includemany of the same references found in the mapping table of version 1604′.As such, when accessing unchanged pages or data objects along the firstbranch, a user assigned to the first branch will actually use the pagesand objects of version 1604′. Likewise, when accessing unchanged pagesor data objects along the second branch, a user assigned to the secondbranch will actually use the pages and objects of version 1604′ as well.By causing different users even from different branches to utilize whatare essentially shared copies of same information, the system is able toefficiently scale in terms of the number of different branches that canbe supported.

As observed in FIG. 15, the distributed database 1513_1 through 1513_Nand snapshot 1515_1 through 1515_N functions of each CNS node 1504_1through 1504_N may be integrated with a “volume manager” function 1516_1through 1516_N that is designed to assign certain users to certainbranches and provide them with (or otherwise cause them to only use) theobject IDs of head objects associated with a specific branch that hasbeen assigned to them. The different groups of users may correspond, forexample, to different functional roles within a corporation, and/ordifferent corporations, etc.

Note that the above examples were directed only to branches from aparticular distributed consistent database. By providing the sametreatment for all distributed consistent databases within a completestorage system (e.g., a large scale file system or thin provisionedblock storage system) groups of users can be provided with their ownisolated snapshots of the entire storage system to which they are ableto make their own isolated changes. Here, as just one example, a firstgroup of users may be given a first branch number from a particularsnapshot level to which all their changes are tracked from any/alldatabase versions at that snapshot level within the entire file system.A second group of users may be given a second branch number from thesame snapshot level to provide separate read-write snapshots of theentire system to the groups of users that are in isolation of eachother. In an embodiment, each such separate read-write snapshot isreferred to as a “reference volume” that is provided by the volumemanager function 1516_1 through 1516_N. In an embodiment, “referencevolumes” may also be provided for a singular, thin provisioned blockstorage file.

Additionally, a “reference volume” may be established as part of theimage of a virtual machine (e.g., in the case where the CNS nodes aresupporting VM hosting). Here, the virtual machine image is designed toinclude the content of a storage system at a particular state and isexpected to be able to operate in isolation to changes made by othervirtual machines who also started with the same initial stateinformation. By assigning each virtual machine a unique branch numberfrom a particular snapshot level, each virtual machine will start withthe content of the snapshot level as its initial state and be able tomake changes to that state along its branch in isolation of changes madeby other virtual machines to that same initial state along their owndedicated branches. The behavior of the virtual machine may beadditionally designed to provided for periodic backups. In this case,each virtual machine is permitted to take snapshots from its own branchon a periodic basis.

The volume manager function 1516_1 through 1516_N (apart from virtualmachine images) may additionally configure snapshots to be read-only forincremental back-ups (e.g., hourly, daily, weekly, etc.) and configuresnapshots that are to be used as “reference volumes” by various users asread-write snapshots. An incremental backup snapshot taken along aparticular branch can be used to “restore” that branch's referencevolume (e.g., in response to a crash) by instilling the storage system'sdistributed consistent databases along the reference volume's branchwith the respective content of these databases from the latest back-up.

Apart from volume management, the relational database functions 1510_1through 1510_N of FIG. 15 may be configured to utilize snapshots so thatmultiple concurrent transactions that run off a same databaseimplemented within KVS can instead be executed in a SERIALIZABLEfashion. For instance, a first snapshot of a database may be taken and afirst transaction may be permitted to execute and commit from the firstsnapshot. A second transaction may be permitted to execute and commitfrom a second snapshot of the database that reflects the state of thedatabase immediately after the first transaction commits.

6.0 More Detailed Embodiment with Gen Numbers

FIGS. 19A through 19I describe some general concepts and methodologiesfor the taking of snapshots and forks for a distributed consistentdatabase within a storage system.

As observed in FIG. 19A, initially a thin provisioned file system isdeployed having snapshot level 0. FIG. 19A pertains to only one of thefiles within the system (file “BAR”). Consistent with the discussionsprovided above, the file BAR may be contained within a folder (e.g.,“FOO”) that is itself implemented as a distributed consistent databasewhose corresponding hierarchy of pages includes a leaf page thatidentifies the object proxy for the file BAR. Here, object proxy 1901corresponds to the object proxy for the file BAR. As observed,initially, the object proxy 1901 only contains a single reference 1902to the active/primary version of the BAR file.

The active/primary version of the BAR file is depicted in FIG. 19A asincluding a head object 1903 having a mapping table 1904. For simplicitythe hierarchy of pages is depicted as including only three leaf pagesP1, P2 and P3 contained in KVS objects 1905, 1906 and 1907,respectively. Leaf page P1 references stripes S1 and S2, leaf page P2references stripes S3 and S4 and leaf page P3 references stripes S5 andS6. Each of the stripes have a generation number of 0. As anoptimization, the object proxy 1901 may be kept in the head object 1903of the active/primary version of the BAR file (so that only one and nottwo KVS accesses are needed to access the active/primary version). Theuse of a generation number (or “gen number”) will be described in moredetail below.

As of the moment depicted in FIG. 19A, the highest gen number reflectedin the active/primary version is 0 and the lowest gen number reflectedin the active/primary version is 0. As such, the gen number is given a“range” of [0,0]. A distributed consistent database whose range includesa set of same numbers corresponds to a distributed consistent databasethat does not represent consolidated snapshots. Consolidated snapshotsare described in more detail further below.

After a brief while, as observed in FIG. 19B, a change is made to theactive/primary version of the BAR file. In particular, stripe S6 isdeleted. Note that each of the entries in the mapping table 1904 aremarked to indicate whether the object containing the page for an entryin the mapping table 1904 can be deleted. Specifically, each entry isgiven a value of “1” which means the respective object of each entry ispermitted to be deleted. As of yet, no snapshots or forks have beentaken of the BAR file which means no current state of the active/primarydatabase needs to be preserved. In response to the deletion of stripeS6, leaf page P3 needs to be changed to now only refer to stripe S5 (andnot S5 and S6).

Thus a new P3 leaf page that only references S5 is created and stored innew object 1908 (that is referenced by key K4′). Furthermore, becausemapping table entries are permitted to be deleted as indicated by theextra bit in the mapping table 1904, the object 1907 that held theoriginal version of P3 (referenced by key K4) is also deleted. The headobject for the active/primary version of the BAR file is stored backinto KVS with the updated mapping table 1904′ that contains thereference to new object 1908 rather than object 1907 for page P3. There-stored head object 1903 with the updated mapping table 1904′ can beviewed as the current active/primary version for the BAR file. The headobject is stored with current tag value of 0 because that is the valueof the snapshot counter when the head object is stored. The head objectalso keeps range [0,0].

After the deletion of S6 is made, referring to FIG. 19C, a global forkis declared for the entire file system of which the BAR file is a part.Note that the global fork is declared before any other forks orsnapshots of the storage system have been taken. In the presentembodiment, unlike some of the embodiments discussed above (e.g., inreference to FIG. 17), the object proxy 1901 does not track versions ofsnapshots but does track the different forks for a particular database.As such, as will become more clear below, object proxy 1901 will containa list of forks that are taken from the BAR file but references tosnapshots taken of the BAR file are contained elsewhere. Also as will bemade more clear below, the global snapshot counter neverthelessincrements on either the taking of a snapshot or a fork because bothcause the creation of a new head object for the BAR file.

In response to the global fork being taken of the entire file system,upon any next change to the BAR file, another new head object 1913 iscopied from the head object 1903 of the active/primary version to createthe “seed” database for the fork. An entry 1910 is created in the objectproxy 1902 for the first fork that references the head object 1913 ofthe seed database for the first fork. The entry is given snapshot level1 because the taking of the global fork caused the global snapshotcounter to increment.

Note that the trigger condition for the creation of seed head 1913 is achange to the BAR file after the global fork is taken. By creating seedhead 1913, the state of the active primary database at the moment of thefork was taken is preserved in head object 1903. The change to the BARfile that triggered the creation of seed head 1913 can be any changesuch as a change that does not change any of the data stripes kept bythe active/primary version of BAR (e.g., movement of the BAR file fromthe FOO folder to another different folder). For ease of drawing such anon-destructive change is assumed, as such no changes to theactive/primary version are observed in FIG. 19C. For additionalsimplicity, subsequent forks described below are assumed to be createdthrough similar processes.

At the initial creation of the fork, the effective content of the seeddatabase for the fork is an exact duplicate of the content of theactive/primary version 1902′ at the moment the fork is taken. Theentries of mapping table 1904′ of the active/primary version, however,are modified to reflect that they can no longer be deleted (because thefork depends on them). Likewise, the entries of the mapping table 1914of the fork are also marked to indicate that they also cannot be deleted(because the active/primary version depends on them). The copied overseed head object 1913 is stored with current tag value of 1 because thesnapshot counter was incremented from 0 to 1 in response to the creationof the fork. Moreover, the seed head object 1913 is given a gen numberrange of [1,1] (which means all the stripes/data objects will be taggedwith a gen number of 1). Note that at this stage the fork database isonly represented by the head object 1913 as it is a total copy of theactive/primary version and does not yet have any modifications specifiedto itself. As such, the fork entirely relies on the objects holding thepages and stripes of the active/primary version.

Subsequent to the creation of the fork, as observed in FIG. 19D, anotherchange is made to the active/primary version. This time the change isthe (re) addition of stripe S6 to page P3. Again a new P3 page iscreated for the active/primary version that references both S5 and S6and a new object 1909, referenced by key K4″, is created to store thatpage. The entry in mapping table 1904′ is updated to reference object1909 instead of object 1908 and the updated mapping table 1904″ is keptin the head object 1903 for the active/primary version. This time,however, because of the markings in the mapping table 1904′, object 1908is not deleted but is preserved in KVS (again, the fork relies on it).Because only the fork now relies on the version of P3 kept at K4′(object 1908), the mapping table 1914 for the fork can be modified toreflect that the fork now owns this version of P3 and is free to deleteit (e.g., upon a next change by the fork to P3). The head object 1903for the current version is stored with a current tag value of 1 becausethat is the value of the snapshot counter when the head object isstored. Moreover, the head object is given a range of [1,1].

After the second change to the active/current version is made, asobserved in FIG. 19E, a change is made to the fork version of the BARfile. Specifically, both stripes S1 and S2 are modified to produceversions of S1 and S2 kept in new objects referenced by K5′ and K6′,respectively. The modification of a data object (such as a stripe)causes its respective gen number to increment. As such, the gen numberfor both S1 and S2 in the fork increment from 0 to 1. Because of themodification of stripes S1 and S2, page P1 needs to be modified tochange its references to new objects for stripes S1 and S2 in K5′ andK6′ and also increment their respective gen numbers from 0 to 1. Thus anew page P1 is created and kept in new object 1920 that is referencedwith key or object ID K2′. The mapping table 1914 within the originalseed head object 1913, as discussed above, originally had the entry forP1 marked to indicate that the object holding P1 in K2 could not bedeleted (see mapping table 1914 of FIG. 19D).

As such the original object 1905 referenced by K2 that contained P1 isnot deleted by the modification to stripes S1 and S2 (here, theactive/primary version 1902 still depends on this version of P3).Nevertheless, the seed head object's original mapping table 1914 ismodified to create a new mapping table 1914′ that references object 1920rather than object 1905. Additionally, the entry in the updated mappingtable for page P1 now indicates that it can be deleted (since no otherversions of the BAR file depend on it). Here, it is recognized that themodification to stripes S1 and S2 are local to the fork and no snapshotsof the fork currently exist. As such, the fork is currently free todelete object 1920. The head object 1913 for the fork having the newmapping table 1914′ is stored in KVS with a current tag of 1 and range[1,1].

After the changes to stripes S1 and S2 are made, as observed in FIG.19F, a global snapshot is taken of the entire storage system. Asdescribed in the preceding section, in response, the snapshot counter isincremented from a value of 1 to a value of 2.

After the global snapshot counter is incremented to reflect the takingof the global snapshot, as observed in FIG. 19G, another change is madeto the fork. Specifically, a modification is made only to stripe S2which creates a new object, referenced by key/object ID K6″, in whichthe new modified version of stripe S2 is kept. As discussed in thepreceding section, the current tag value (1) of the head object 1913 forthe fork is compared against the value of the snapshot counter (2).Because they are different, the head object 1913 is stored in KVS withits mapping table 1914′ to preserve the state of the fork at the momentthe snapshot was taken. That is, the head object 1913 having a range of[1,1] is preserved in KVS.

A second head object 1923, referenced by key/object ID K12, is createdto represent the new “head” of the fork which includes a mapping table1924 that is initially identical in content to its parent mapping table1914′ in terms of the objects that it refers to. However, the initialmarkings of the entries on the mapping table 1924 are modified whereappropriate to reflect that none of them may be deleted (because theparent version 1913 now relies on them). The second head object 1923also contains a reference 1930 to parent head object 1913 to “chain” thedifferent snapshot versions of the fork together in a backwardreferenced fashion. Also, entry 1910 in the object proxy 1901 ismodified to reflect that the new head of the fork is kept in the headobject 1923 referenced by K12 (alternatively, the new head object 1923can be given key K11 and the parent head object 1913 can be given newkey K12 thereby not requiring any change to the proxy 1901). Should auser seek the snapshot level of the first snapshot taken of the fork,represented by head object 1913, the user's access will first bereferenced to head object 1923 in K12 by way of the object proxy andwill then referenced back to the correct head object 1913 throughreference 1930.

The modification to stripe S2 along the fork additionally includes thefollowing activities. A new object, referenced by K6″ is created tostore the new version of S2 and a new page P1 is created that referencesnew object K6″ and increments the gen number for S2 from 1 to 2. Anothernew object 1926, referenced by K2″, is created to store the new versionof P1. The mapping table 1924 of the new fork head object 1923 isamended to reference the new object 1926 that contains the new versionof P1. Because the mapping table 1924 originally indicated that none ofits entries may be deleted, the object that contains original page P1 inK2′ is not deleted (because the first snapshot depends on it). Howeverwhen the mapping table 1924 is modified to replace the entry for P1 withthe new object 1926, the entry for P1 is marked to indicate that it canbe deleted as no other versions of the BAR file depend on it. Likewise,the entry for P1 in mapping table 1914′ that references K2′ may also bemodified to reflect that the first snapshot level may delete it since noother version depends upon it. The second head object 1923 has a currenttag of 2 and a range of [2,2].

If a second global snapshot were to be taken at this point and then thefirst snapshot level were to be consolidated into the second snapshotlevel (e.g., the first snapshot level is old/stale enough and can be ridoff, however, the second snapshot level still depends on its objects),head objects 1913 and 1923 could be “merged” to form a head object(which can remain in object 1923) now having range [1,2] to reflect themerger and providing the head object 1923 with ownership of bothversions of P1 (referenced by K2′ and K2″). The range information [1,2]can be used, e.g., for subsequent snapshot requests to recognize whichsnapshot levels have been consolidated into object 1923. Because newhead object 1923 has ownership of both versions and the first snapshotlevel has been removed, object 1920 referenced by K2′ can deleted. Theremainder of the discussion will assume that neither the second snapshotnor the merger occurred.

After the head object 1923 has been stored in KVS, a “local” fork fromhead object 1923 is taken. That is, a second fork from the first fork istaken that is specific to the BAR file and is not the result of a“global” fork taken across the entire storage system. Because the forkis a local fork there is no increment of the global snapshot counterassociated with the local fork (here, a special “local fork bit” may beset to trigger creation of the local fork upon a next change to the BARfile). Referring to FIG. 19H the object proxy 1901 is therefore modifiedagain to include a reference 1931 to the new fork which is realized withthe creation of new head object 1927, referenced by key K13, thatcontains a mapping table 1928 whose page to object references areidentical to mapping table 1924. The entries of both of mapping tables1924 and 1928 are marked to indicate that their respective pages cannotbe deleted because the first fork and second forms depend on them.

As observed in FIG. 19I a modification is made to the second fork'sversion of S2 which creates another version of S2 kept in K6′″. Here,the mapping table 1928 of the head object 1927 for the second fork ismodified to include an entry for a new page P1 that is kept in a newobject 1929. The new page P1′ refers to a new data object K6′″ thatcontains the new version of S2 and having a gen number that has beenincremented from 2 to 3. The original object 1926 containing the secondfork's initial version of page P1 in object 1926 is not deleted becauseof the markings originally found in table 1928 that indicated no pagescould be deleted. However, the newly created entry in mapping table 1928that refers to object 1929 for page P1 is marked to indicate that it canbe deleted as no other version of the BAR file depends upon it. The headobject 1927 for the second fork is stored in KVS with range [3,3].

Note that a “local snapshot” can effectively be implemented by forming alocal fork and then not modifying it (e.g., by writing a change to it).This can be done in various ways such as simply creating a local forkand not permitting any children to derive from it. Alternatively, achild may be created from the local snapshot which acts as a seed (bywriting a modification) but the child object has a reference, similar tothe relationship between objects 1923 and 1913, back to the seed localfork. The object proxy in an embodiment only refers to the youngestchild from the seed to keep the size of the object proxy manageable. Thechildren back reference to one another to rely on the objects of theirparent(s). By chaining through the back references the seed/snapshotview can be obtained. In an embodiment, the local snapshot level orlocal snapshot count is tracked by reference to the local forkidentifiers (e.g., the second “1” in the notation fork_1_1 in reference1931 of the object proxy).

Additionally it is pertinent to recognize that with respect to any ofthe various methodologies and processes described above, after an objectis called up from KVS and subsequently manipulated or otherwise used insystem memory it may be kept in system memory (e.g., as a form ofcaching layer above KVS) or may be written back to KVS aftermanipulation is made to it or it is otherwise used. Exactly whatpolicies apply that dictate when an object called up from KVS is to bewritten back to KVS from system memory after manipulation is made to itor it is otherwise used may vary from embodiment depending on factorssuch as the size of system memory, the number of objects in systemmemory, priorities given to threads or applications running themethodologies/processes, the length of time an object has remained insystem memory without accessed made to it, etc. Thus, with respect tothe processes described above objects may be written back to KVS fromsystem memory after being called up from KVS according to a wide rangeof possibilities and/or factors.

Additionally it is pertinent to recognize that with respect to any ofthe various methodologies and processes described above, after an objectis called up from KVS and subsequently manipulated or otherwise used insystem memory it may be kept in system memory (e.g., as a form ofcaching layer above KVS) or may be written back to KVS aftermanipulation is made to it or it is otherwise used. Exactly whatpolicies apply that dictate when an object called up from KVS is to bewritten back to KVS from system memory after manipulation is made to itor it is otherwise used may vary from embodiment depending on factorssuch as the size of system memory, the number of objects in systemmemory, priorities given to threads or applications running themethodologies/processes, the length of time an object has remained insystem memory without accessed made to it, etc. Thus, with respect tothe processes described above objects may be written back to KVS fromsystem memory after being called up from KVS according to a wide rangeof possibilities and/or factors.

Note that the comments made above with respect to FIGS. 13 and 14concerning platform level details also apply to any of the global orlocal snapshot or branching/forking operations or other approachesdiscussed above in Sections 5.0 and 6.0 with respect to FIGS. 15 through19, such as the CNS nodes depicted in FIG. 15 and any of the snapshotand/or branching or other processes they may perform as discussed abovein Sections 5.0 and 6.0.

In the foregoing specification, the invention has been described withreference to specific exemplary embodiments thereof. It will, however,be evident that various modifications and changes may be made theretowithout departing from the broader spirit and scope of the invention asset forth in the appended claims. The specification and drawings are,accordingly, to be regarded in an illustrative rather than a restrictivesense.

The invention claimed is:
 1. An apparatus comprising one of morecomputers, said apparatus comprising: a) an interface comprised of firstmachine executable instructions that when processed by at least oneprocessor of said one or more computers causes the apparatus to accessan object store having a plurality of database object groups toimplement a storage system, each of said database object groupsrespectively comprising the following: a first plurality of immutableobjects containing information kept within said database; a secondplurality of immutable objects containing a hierarchy of pages foraccessing said first plurality of immutable objects, wherein, a subsetof said pages are called up from said storage system and processed bysaid at least one processor of said one or more computers in ahierarchical order in order to access a particular one of said firstplurality of objects; a mutable object containing a mapping table thatmaps identifiers of said pages to identifiers of objects of said secondplurality of immutable objects; b) snapshot logic composed of secondmachine executable instructions that when processed by at least oneprocessor of said one or more computers cause said apparatus to take asnapshot of one of said database object groups such that a respectivestate for said one of said database object groups when said snapshot istaken is preserved.
 2. The apparatus of claim 1 wherein said storagesystem is presentable to a user as a file system.
 3. The apparatus ofclaim 2 wherein said database object group is used to implement a fileof said file system.
 4. The apparatus of claim 1 wherein said storagesystem is presentable to a user as a thin provisioned block storagesystem.
 5. The apparatus of claim 4 wherein one of said first pluralityof immutable objects is used to implement a stripe of said thinprovisioned block storage system.
 6. The apparatus of claim 1 whereinsaid mutable object contains first information that indicates which ofsaid second plurality of immutable objects are delete-able.
 7. Theapparatus of claim 6 wherein said snapshot logic is to update said firstinformation upon a next change to said one database object group aftersaid snapshot is taken, said update indicating that said secondplurality of immutable objects are not delete-able.
 8. The apparatus ofclaim 7 wherein said snapshot logic is to re-update said firstinformation that a new changed version of one of said second pluralityof objects is delete-able, said first information kept with a snapshotversion of said one database object group.
 9. The apparatus of claim 1wherein said snapshot logic is to implement a fork of said databaseobject group by permitting changes to be made to said snapshot of saiddatabase object group.
 10. The apparatus of claim 1 wherein saidsnapshot is implemented as a branch from said one database object groupthat is not written to.
 11. A method, comprising: copying information ofa first database object group as part of forming a second databaseobject group, said second database object group representing a seed fora fork within a storage system composed of multiple database objectgroups, wherein said information identifies pages and their respectivekeys shared by both said first and second database object groups andthat the same cannot be deleted; accessing first data in said firstdatabase object group by calling and processing certain ones of saidpages in a hierarchical order; accessing a second version of said firstdata in said second object group by calling and processing said certainones of said pages in said hierarchical order; in response to making achange to said fork, changing said second database object group'sversion of said information to indicate that a leaf page that wasmodified as a consequence of said change can be deleted.
 12. The methodof claim 11 further comprising taking a snapshot of said storage system.13. The method of claim 11 further comprising taking a snapshot only ofsaid first database object group.
 14. The method of claim 13 whereinsaid snapshot is implemented by not writing to said fork.
 15. The methodof claim 11 wherein said fork is a global fork of said storage system.16. The method of claim 11 wherein said fork is a local fork of saidfirst database object group.